#24 Hide context menus quicker

[paaguti]

I have submitted a patch to GIMAGEVIEW which allows to
identify external programs, which that the thumbnail(s)
of the processed images be refreshed automatically
after their execution.

In the current implementation, the context menus stay
on the screen until the point, where the external
programs have been executed. Would it be possible to
hide the context menu right after it has been clicked.

It would allow to use the status bar for user feedback
and the general impression for the user would be smoother.