Generalize support for shell's schema overrides

GNOME Shell overrides some settings to use different default values
than standalone mutter. With the introduction of classic mode, things
got a little more complicated, as some of the overrides were reverted
again by extensions - to support either mode, we needed to pick the
right schema for all affected settings.
The shell has now introduced mode-specific overrides, and classic mode
started to use that instead of extensions; this means we will now need
to adapt all settings that *both* normal and classic mode override.
However rather than adjusting the affected settings, the new approach
allows us to generalize the support for schema overrides depending on
the current mode - this should allow us to handle future changes to
overrides in either normal or classic mode automatically.

https://bugzilla.gnome.org/show_bug.cgi?id=701760
Updated German translation
Support using schema paths in GSettings

https://bugzilla.gnome.org/show_bug.cgi?id=699298
Add Wacom group and tweak for turning on/off tablet PC mode

https://bugzilla.gnome.org/show_bug.cgi?id=699298
fix missing schema check