ivi-controller, ilmControl: Add ilm_SetKeyboardMultiFocus

This adds the function
ilm_SetKeyboardMultiFocus(t_ilm_surface *pSurfaceIds, t_ilm_int number)

which takes an array (and a size) of the surface IDs that should
be focussed.
The old ilm_SetKeyboardFocusOn() has been preserved for the sake of
backwards compatibility.
LayerManagerControl: Add 'set keyboard focus' command

The full command is "LayerManagerControl set keyboard focus <idarray>"
The old command, "LayerManagerControl set <surfaceid> keyboard focus"
has been preserved for backwards compatibility
ilmControl, LayerManagerControl: Add ilm_GetKeyboardMultiFocusSurfaceIds and make LayerManagerControl use it

The function ilm_GetKeyboardFocusSurfaceId is retained for backwards
compatibility.
LayerManagerControl: Make surface printing recognize multi-surface focus
test: Add multi-surface focus test

This test works in isolation, but due to incomplete teardown
between tests, it does not work when run alongside other tests.