If a `bindsym` config directive specifies a symbol beginning with
"button", the binding will be given the type B_MOUSE for the indicated
button number.
Example:
bindsym $mod+button2 exec echo 'button two'
This will be interpreted as having input code (now `keycode`) 2 and type
B_MOUSE.
The mechanism to find and run mouse bindings on mouse events is not
implemented.
Rename `get_binding` to `get_keyboard_binding` and ensure that this
function only accesses bindings of type B_KEYBOARD. Other types of
bindings (e.g. mouse bindings) will be accessed by a different function.
Create files bindings.[ch] to contain functions for configuring,
finding, and running bindings.
Use the new function `configure_binding` for binding configuration. This
function adds a binding from config parameters.
Export the function `modifiers_from_str` from config_directives.h.
This change is made in preparation for the new bindmouse functionality.
