In certain circumstances, the keyboard modifiers get out-of-sync between the
guest and the client. This is easy to reproduce with the following steps:
 - launch virt-viewer with a guest that is not running
 - start the guest
 - while guest is booting, enable CAPS LOCK on the client
 - after guest finishes booting, it will set its modifiers to a default value
   (e.g. numlock on, capslock off)
 - now capslock is OFF in the guest, but ON in the client
 - toggle caps lock
 - now capslock is ON in the guest, but OFF in the client

This moves the responsibility for synchronizing client and guest modifiers into
SpiceGtkSession. It can't be handled easily within the SpiceDisplay widget since
there can be multiple display widgets for each inputs channel.

A new function (spice_gtk_session_sync_keyboard_modifiers()) was added so that
synchronization can be triggered manually if desired. But it also registers a
signal handler for the InputsChannel::inputs-modifiers signal to detect when the
guest has changed its modifiers. The signal handler simply overrides the guests
modifiers and sets them back to the value from the client.