Revert "Automatically stop watchers whose element is destroyed"

This reverts commit fd172ef6641887618f05cb7e34a87cd345a4ee61.

There are problems with this commit and a different approach is
necessary.
Save window pid and id on creation

Rather than cache the id and find the pid again every time. This is
important for processing events on windows which were deleted, since
this information is no longer available for them.
Track watchers and automatically stop them in lua

Automatically stop watchers whose element is destroyed, take 2. This
works with applications, too, and doesn't have any weird behavior.