ballooneval is a global option, so setting it while initialising tagbar
permanently enables ballooneval functionality for the GUI, overriding
the vim default (disabled) or a user explicitly setting noballooneval.
To address this, only set ballooneval when entering the tagbar window,
and restore the original ballooneval setting when leaving the window.
This is not perfect, but there is precedent, Netrw behaves like this.
ballooneval is a global option, so setting it while initialising tagbar permanently enables ballooneval functionality for the GUI, overriding the vim default (disabled) or a user explicitly setting noballooneval.
To address this, only set ballooneval when entering the tagbar window, and restore the original ballooneval setting when leaving the window.
This is not perfect, but there is precedent, Netrw behaves like this.
Fixes #499