boxpointer: Compute source allocation and monitor just once

Compute the source actor monitor and allocation when repositioning and keep it
cached so that we've not to calculating it again in _calculateArrowSide.

Since _calculateArrowSide only is called inside _updateFlip that is always
called just after reposition, we can be sure that the computed values are still
correct.

GNOME/gnome-shell!435
4 jobs for !435 with boxpointer-real-positioning in 5 minutes and 46 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #239689
check_commit_log

00:01:40

 
  Build
passed #239690
build

00:02:54

 
  Test
passed #239691
test

00:00:47

passed #239692
test-pot

00:01:11