Memory leak when creating / destroying inline instances

[dvotion]

When you dynamically create inline instances and then destroy an instance with instanceName.destroy() the mutation observers aren't destroyed and thus causing some serious memory leak and dom leakage.

On a page with multiple instances and a lot of user interaction this leak quickly starts to grow big (multiple mb's per step) and causing the ckeditor / browser to become really slow.

I've attached a screenshot of a heap snapshot comparison done in Chrome. The steps I did were:

ActionSteps = create inline instance and then destroy it

1. do: ActionSteps
2. take Heapsnapshot
3. do: ActionSteps
4. take Heapsnapshot
5. do: ActionSteps
6. take Heapsnapshot

Then select heapsnapshot 3 and select from dropdown "Object allocated from snapshot 1 and 2"

[Jakub Ś]

Other related issues: #16869, #12307, #11010, #13123.