dnd: Use pointer actor instead of picking manually

Use the pointer actor for getting the drop target and currently hovered
actor, it's available anyway and is a lot faster than picking through
all actors on stage.

This allows removing the idle loop we currently use to handle the
performance impact of picking.

GNOME/gnome-shell!530
4 jobs for !530 with dnd-use-pointer-actor in 10 minutes and 49 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Review
passed #290055
check_commit_log

00:00:33

 
  Build
passed #290056
build

00:08:44

 
  Test
passed #290057
test

00:01:21

passed #290058
test-pot

00:01:30