Settings Editor does not dispose properly

[bpasero]

I am testing from the ben/editor branch that contains the changes for Grid. If you split an editor to the side and then close that editor, we now actually dispose the editor and no longer keeping it around as we used to before. This means that we now rely on the editor freeing up its resources properly.

For the new settings editor I see lots of controls still hanging around after:

• open a single settings editor
• split it once
• close the second instance
• create a heap snapshot
• repeat split/close a couple of times
• create a heap snapshot again
• compare it to the first