Revert "Fix GTK frame clock warnings with our about dialog"

This reverts commit ec7071062c5dc39d5f893a830b3c26968a579251.

Apparently the selected state of an event box doesn't necessary mean it
will be styled with a selection background color.  This e.g. isn't the
case in GTK 3.14's Adwaita (default) theme.

So, revert the commit that removed custom style overrides.  Next commit
will fix the GTK 3.10 problem this reverted commit fixed.
Fix custom about dialog style overriding

Don't assume changing the style inside a style-set handler results in
recursion, as it would be wrong if the next signal emission was
triggered asynchronously.  Instead, only modify the style if it isn't
already as we want it.

This fixes the about dialog's style-set handlers on GTK 3.10 which
didn't liked constant style updating.
GTK: Don't hardcode auto-completion popup frame border width
Merge branch 'about-banner-fix'
Merge branch 'scintilla/autoc-sizing'

Backport changes applied upstream not to hard-code auto-completion popup
frame border size.