Refactor fix for double }} in completion

[jlelong]

This commit basically reverts #2293 and #2080. Instead it uses the same approach as #2370.
The new approach is less intrusive and much cleaner.
@tamuratak can you test it?

[tamuratak]

The issue #2075 occurs again.

If you are saying that stopping auto-closing with } for \begin{ is enough, I agree it is enough.

[jlelong]

#2075 was related to the auto-closing of {. With this PR, typing \begin{item and completing works fine

However, it is true that completing \begin{item} fails

But I cannot really see the purpose of such as completion. Instead,

What could be done when completing \begin{item} with the cursor inside {} is to only complete the environment name and not to add the \end{} part.

Are you fine with this approach?

[tamuratak]

Yes, I am fine with it.