month-view: fix ::forall and ::remove impl

Take care of removing parts-widgets first whenever a master widgets is removed
month-view: optimize text direction query

By keeping the value of the direction factor k around we save code and
function calling every time we need to use it.
month-view: queue draw on size_allocate

It should not be necessary to queue a draw operation after
::size_allocate. This has been discussed over with Gtk+ devs, as soon as
the bug is fixed these statements will goes away.