Over time the number of integration tests that get the rectangle for a
given selector has increased quite a bit, and the code to do so has
consequently become duplicated.
This commit refactors the integration tests to move the rectangle
fetching code to a single place, which reduces the code by over 400
lines and makes the individual tests simpler.
The original bug was because the parent was null when trying to show
an highlight annotation which led to an exception.
That led me to think about having some null/non-null parent when removing
an editor: it's a mess especially if a destroyed parent is still attached
to an editor. Consequently, this patch always sets the parent to null when
deleting the editor.
