Previously, there would be globally just a capture and a bubble phase,
with the event just going down the hierarchy once, and the up once.
GTK_PHASE_TARGET actually meaning "run within event handlers", so in
a hierarchy of 3 widgets, emission would be:

  Capture(C)
  Capture(B)
  Capture(A)
  Target(A) (if event handlers allow)
  Bubble(A)
  Target(B) (if event handlers allow)
  Bubble(B)
  Target(C) (if event handlers allow)
  Bubble(C)

This commit changes this behavior and uses GTK_PHASE_TARGET in a less
misleading way, running only on the widget that was meant to receive
the event. And GTK_PHASE_BUBBLE has taken over the execution place of
GTK_PHASE_TARGET, so the emission remains:

  Capture(C)
  Capture(B)
  Capture(A)
  Target(A)
  Bubble(A) (if event handlers allow)
  Bubble(B) (...)
  Bubble(C) (...)

As it was, GTK_PHASE_BUBBLE was useful for running event controllers
paralelly to event handlers, without modifying a single line in those.
For those mixed scenarios, Any of the other phases will have to be
used at discretion, or the event handlers eventually changed to chain
up and let the default event handlers in GtkWidget to be run.