Remove the Alert popups from the graphics cache.

Remove the Alert popups from the graphics cache.
Sometimes, these Alerts are displayed with only part of the background
greyed out, which smacks of "wierd xforms bug".

I can obtain this when loading a buffer from the command line "lyx buffer.lyx".
It appears that xforms isn't happy if the goodie is displayed before the
main window. Just a guess though...

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@4007 a592a061-630c-0410-9148-cb99ea01b6c8