display.c: fix scroll-zoom modifier mask
Revert old steam fix for notifications (it actually causes them to
block interaction with the desktop now during a notification)

Fixes linuxmint/Cinnamon#3718

This reverts (most of) commit 8f2952bd6e32eeed5674c25fea34497e69074c84.
display.c: don't allow unmodified scroll-to-zoom
Fix keybindings