backend-x11: Only spoof the event when we're passing it to Clutter

There's no need to do so otherwise.
backend-x11: Remove pointless goto

The function is not as complex as needing that, plus it always jumped
anytime bypass_clutter is TRUE, so make all conditional code depend on
that.
backend-x11: Ensure touch event coordinates are translated to the stage's

Now that those events are going to be received, coordinates translation
should also happen on those.
ui: Remove unneeded code

There's no way now that GDK will hold a grab, so it is safe to remove
this now.
frame: Only select for ButtonPress/Release/Motion/Crossing

Touch events are largely ignored on GdkEvent emulation, so only
make frames receive pointer events, only the pointer emulating
touch will be reported, and any other further touches will be
ignored, which is about the behavior we want. This makes window
dragging possible again on touch.
ui: Remove XI_TouchBegin handling from window frame event handlers

This is now unnecessary as only pointer events are selected, so pointer
emulation will take care of sending only pointer events.
events: Update current time on CLUTTER_TOUCH_BEGIN events

Just as with key/button press events, update the current interaction
time when a touch begins on a window.
compositor: emit grab-op-begin/end signals on plugin grabs

This makes it possible to track there grabs triggered this way, in
addition to the ones handled by mutter.