Only show appliable scales

This superseeds MR !314 (closed).

Main differences are:

• Port to new widget (requires fewer code changes)
• Rename and modify "is_scale_allowed_by_all_monitors" to "is_scale_allowed_by_active_monitors"
• Drop is_mirroring_configured addition and check for cloning mode instead

CC: @robert.ancell, @3v1n0