window: fix alt-left/right keyboard shortcuts

The call to gdk_keymap_translate_keyboard_state doesn't seem to really
do much here, except change the way we have to test for Ctrl+Shift+T by
consuming GDK_SHIFT_MASK. In particular, the keyval returned is already
the keyval in the key event, so that's not really doing anything for us.
But this is what the GDK documentation tells us to do, so why not. The
important change here, as shown in the GDK documentation, is to mask out
all the modifiers except Ctrl/Shift/Mod1 so that we ignore virtual
modifiers like Meta that we really don't want to see.

This mostly but not completely fixes the bug where the alt-left/right
shortcuts for back/forward stop working. It fixes the regression where
these shortcuts stopped working after I introduced key event filtering.
It *does not* fix the issue that was originally reported, which is that
the shortcuts mysteriously stop working sometimes; it cannot fix that
original bug, because the key event filtering was not added until...