Rework pause functionality, closes #177

The current pause functionality simply removes all the autocommands to
stop updating Tagbar. This has the problem that at the moment the
statusline functions immediately restore the commands so pausing doesn't
work, but even if that didn't happen it would stop the statusline
functionality from working which is not desirable. The solution is to
have a reference to the paused file which will get used by the Tagbar
window, but not the statusline functions.