widget: Disable more widget keybindings

Disable all of GtkWidget's keybindings except the ones for popup-menu.
We do not want that either, but handling Shift-F10/MenuKey ourself will
require new API to en/disable it, so defer that for now.

This reverts commit 46b7464576141d69bd0dce16409141cfb26a8225, wontfixing
bug https://bugzilla.gnome.org/show_bug.cgi?id=153265.

https://bugzilla.gnome.org/show_bug.cgi?id=726438