Implement hooks for viewers

This implements capability for view and callback hooks for all viewers (though
not all viewers have callback functionalities implemented).

For Zathura and MuPDF this change also implements a default focus behaviour
which may now be changed through the use of these hooks.

autoload/vimtex/view.vim

@@ -52,6 +52,18 @@ function! vimtex#view#init_buffer() " {{{1
   execute 'let b:vimtex.viewer = ' . viewer
   call b:vimtex.viewer.init()
 
+  "
+  " Create view and/or callback hooks (if they exist)
+  "
+  for hook in ['view', 'callback']
+    execute 'let hookfunc = ''*''
+          \ . g:vimtex_view_' . g:vimtex_view_method . '_hook_' . hook
+    if exists(hookfunc)
+      execute 'let b:vimtex.viewer.hook_' . hook . ' = function(g:vimtex_view_'
+            \ . g:vimtex_view_method . '_hook_' . hook . ')'
+    endif
+  endfor
+
   "
   " Define commands
   "
@@ -98,6 +110,10 @@ function! s:general.view(file) dict " {{{2
   let exe.cmd .= ' ' . vimtex#util#fnameescape(outfile)
   call vimtex#util#execute(exe)
   let self.cmd_view = exe.cmd
+
+  if has_key(self, 'hook_view')
+    call self.hook_view()
+  endif
 endfunction
 
 " }}}2
@@ -125,6 +141,14 @@ function! s:mupdf.init() dict " {{{2
     call vimtex#echo#warning('vimtex viewer MuPDF requires xdotool!')
   endif
 
+  "
+  " Default MuPDF settings
+  "
+  call vimtex#util#set_default('g:vimtex_view_mupdf_hook_callback',
+        \ 's:focus_vim')
+  call vimtex#util#set_default('g:vimtex_view_mupdf_hook_view',
+        \ 's:focus_viewer')
+
   let self.class = 'MuPDF'
   let self.xwin_id = 0
   let self.xwin_exists = function('s:xwin_exists')
@@ -144,6 +168,10 @@ function! s:mupdf.view(file) dict " {{{2
   else
     call self.forward_search(outfile)
   endif
+
+  if has_key(self, 'hook_view')
+    call self.hook_view()
+  endif
 endfunction
 
 " }}}2
@@ -235,7 +263,9 @@ function! s:mupdf.latexmk_callback() dict " {{{2
     if self.xwin_get_id()
       call self.xwin_send_keys(g:vimtex_view_mupdf_send_keys)
       call self.forward_search(b:vimtex.out())
-      call self.focus_vim()
+      if has_key(self, 'hook_callback')
+        call self.hook_callback()
+      endif
     endif
   endif
 endfunction
@@ -270,6 +300,10 @@ function! s:okular.view(file) dict " {{{2
   let exe.cmd .= '\#src:' . line('.') . vimtex#util#fnameescape(expand('%:p'))
   call vimtex#util#execute(exe)
   let self.cmd_view = exe.cmd
+
+  if has_key(self, 'hook_view')
+    call self.hook_view()
+  endif
 endfunction
 
 " }}}2
@@ -294,6 +328,10 @@ function! s:qpdfview.view(file) dict " {{{2
   let exe.cmd .= ':' . col('.')
   call vimtex#util#execute(exe)
   let self.cmd_view = exe.cmd
+
+  if has_key(self, 'hook_view')
+    call self.hook_view()
+  endif
 endfunction
 
 " }}}2
@@ -317,6 +355,10 @@ function! s:sumatrapdf.view(file) dict " {{{2
   let exe.cmd .= ' ' . vimtex#util#fnameescape(outfile)
   call vimtex#util#execute(exe)
   let self.cmd_view = exe.cmd
+
+  if has_key(self, 'hook_view')
+    call self.hook_view()
+  endif
 endfunction
 
 " }}}2
@@ -334,12 +376,18 @@ function! s:zathura.init() dict " {{{2
     call vimtex#echo#warning('vimtex viewer Zathura requires xdotool!')
   endif
 
+  "
+  " Default Zathura settings
+  "
+  call vimtex#util#set_default('g:vimtex_view_zathura_hook_callback',
+        \ 's:focus_vim')
+  call vimtex#util#set_default('g:vimtex_view_zathura_hook_view',
+        \ 's:focus_viewer')
+
   let self.class = 'Zathura'
   let self.xwin_id = 0
   let self.xwin_get_id = function('s:xwin_get_id')
   let self.xwin_exists = function('s:xwin_exists')
-  let self.focus_vim = function('s:focus_vim')
-  let self.focus_viewer = function('s:focus_viewer')
 endfunction
 
 " }}}2
@@ -352,6 +400,10 @@ function! s:zathura.view(file) dict " {{{2
   else
     call self.forward_search(outfile)
   endif
+
+  if has_key(self, 'hook_view')
+    call self.hook_view()
+  endif
 endfunction
 
 " }}}2
@@ -379,7 +431,6 @@ function! s:zathura.forward_search(outfile) dict " {{{2
   let exe.cmd .= ' ' . vimtex#util#fnameescape(a:outfile)
   call vimtex#util#execute(exe)
   let self.cmd_forward_search = exe.cmd
-  call self.focus_viewer()
 endfunction
 
 " }}}2
@@ -387,7 +438,9 @@ function! s:zathura.latexmk_callback() dict " {{{2
   if !self.xwin_exists()
     if self.xwin_get_id()
       call self.forward_search(b:vimtex.out())
-      call self.focus_vim()
+      if has_key(self, 'hook_callback')
+        call self.hook_callback()
+      endif
     endif
   endif
 endfunction
@@ -466,6 +519,10 @@ function! s:xwin_send_keys(keys) dict " {{{1
 endfunction
 
 " }}}1
+
+"
+" Hook functions (used as default hooks in some cases)
+"
 function! s:focus_viewer() dict " {{{1
   if !executable('xdotool') | return | endif