core: Unset "pointer emulating" sequence after event processing

The set/unset branches of meta_display_update_pointer_emulating_sequence()
have been split and put directly where it makes sense. The pointer emulated
sequence will be updated before processing the CLUTTER_TOUCH_BEGIN, and
after processing the CLUTTER_TOUCH_END, this way the checks on this hold
true during all the sequence lifetime.

https://bugzilla.gnome.org/show_bug.cgi?id=738411