media-keys: Handle 'Above_Tab' fake key

Mutter supports the 'Above_Tab' fake keysym that refers to the key
that is physically located above the tab key. It is used in the default
shortcut of the "Switch windows of an application" action.

As gtk_accelerator_parse() doesn't recognize the keysym, we display
the shortcut incorrectly as "disabled", and it is not taken into account
for conflict resolution.

Address this by translating binding that contains the 'Above_Tab' string
to bindings where the string is replaced with each possible keysym that
corresponds to the fixed keycode of KEY_GRAVE + 8.

GNOME/gnome-control-center#581
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