plugins/default: Get default keymap from localed

When starting standalone mutter and running using the native backend, we always fall back on using the us pc105 keyboard layout. This can be very frustrating if one is used to using some other keyboard layout, such as dvorak, causing keyboard fumbling everytime when doing something with standalone mutter.

Avoid this involuntary fumbling by having the default plugin query localed what layout the user has actually configured the machine to operate using. It doesn't add any keymap selection user interface, so it'll always use the first one it encounters.