Try harder to discriminate Shift-F10 and F10

Try harder to discriminate Shift-F10 and F10

A change in xkeyboard-config 2.4.1 made it so that function keys
now have a shift level which has the same symbol, but 'eats' the
shift modifier. This would ordinarily make it impossible for us
to discriminate between these key combinations.

This commit tries harder to discriminate in 2 ways:
- XKB has a mechanism to tell us when a modifier should not be
  consumed even though it was used in determining the level.
  We now respect such 'preserved' modifiers. This does not fix
  the Shift-F10 vs F10 problem yet, since xkeyboard-config does
  not currently mark Shift as preserved for function keys.
- Don't consume modifiers that do not change the symbol. For
  the function keys, the symbol on the shift level is the same
  as the base level, so we don't consider Shift consumed.

For more background on the xkeyboard-config change, see
https://bugs.freedesktop.org/show_bug.cgi?id=45008

https://bugzilla.gnome.org/show_bug.cgi?id=661973