[Bug] MultiMask fully refreshes during map summary polling

[mradamcox]

Describe the problem

While auto-refreshing is happening in the map overview (which is enabled if any session is in progress within the map documents), if a new layer has been created, then the entire MultiMask component is re-initialized, meaning any unsaved work on a mask will be lost (very bad). This needs to be handled better, and here are a couple of idea for how to do it:

1. When there is a change in the item_lookup of the Map object, then add a button "new layers available" that would allow the user to chose when the MultiMask should be updated
2. A new layer is just "slipped" into the interface without reinitializing the whole thing. The same strategy for this would be used to handle the bug described in [parker's ticket 2]

Related note: I would like to redesign the layout of this entire page in the future, but this issue should be handled even before that happens.

Expected behavior

Don't lose work in the MultiMask when the map summary page refreshes.