The 'hidden' option is a global option, not a buffer one. If this option is
false, we should check if the 'bufhidden' option, which is local to the buffer,
is set to 'hide'. If so, the buffer can be hidden despite the 'hidden' option
being false.
Bring fully asynchronous completion by polling for completions with a timer
then calling completefunc once the completions are ready. Use the start column
returned by the server in completefunc. Immediately display the last completion
on the TextChangedI event to prevent the popup menu disappearing while waiting
for the completions. Handle the TextChangedI event not being triggered while
the completion menu is open by closing the menu when inserting a character
through the InsertCharPre event, and when deleting a character on the <BS> and
<C-h> keys.
