evdev: Flush event queue before removing an input device

libinput_suspend() will trigger the removal of input devices, but also
the emission of button/key releases pairing everything that is pressed
at that moment. These events are queued, but the ClutterInputDevice
pointers in these will point to invalid memory at the time these are
processed.

Fix this by flushing the event queue, in order to ensure there are no
unprocessed input events after libinput_suspend().

https://bugzilla.gnome.org/show_bug.cgi?id=738520
main: Don't update the PangoContext in clutter_set_font_flags

clutter_set_font_flags already calls clutter_backend_set_font_options,
which emits a signal which our PangoContext listens to, so this is just
duplicate and unneeded code.

https://bugzilla.gnome.org/show_bug.cgi?id=739050
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
Revert "actor: Plug a leak in the implicit transition removal"

This reverts commit 158af1ff594d8984b59dcf90654ed04cd8c53e16.

This commit introduced a regression, so the leak will have to be fixed
in another way.