Fix keyboard layout switching in the greeter

[FreddyFunk]

Fixes #461
Fixes #360

• Keep the layout dropdown in sync after a switch when using cosmic-greeter after locking an already running session by refreshing on runtime xkb config changes (At lock screen, changing keyboard layout with keyboard shortcut does not update keyboard layout drop-down #461).
• Make Super+Space work in the greeter ([FEATURE REQUEST] change the keyboard layout by keyboard shortcut #360): the greeter now serves the InputSourceSwitch D-Bus method itself and rotates the layout.

Super+Space is dispatched by cosmic-comp to com.system76.CosmicSettingsDaemon, which doesn't run in the greeter. So on a fresh boot with no prior login the shortcut hit nothing. The greeter now owns that name and handles the call itself.

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