actor: Create a PangoContext per actor

For a variety of complicated reasons, ClutterText currently sets fields
on the PangoContext when creating a layout. This causes ClutterText to
behave somewhat erratically in certain cases, since the PangoContext is
currently shared between all actors.

GTK+ creates a PangoContext for every single GtkWidget, so it seems like
we should do the same here.

Move the private code that was previously in clutter-main.c into
clutter-actor.c and clean it up a bit. This gives every actor its own
PangoContext it can mutilate whenever it wants, at its heart's content.

https://bugzilla.gnome.org/show_bug.cgi?id=739050