UI: Fix a bunch of memory leaks; rename a few member variables

* A bunch of UI elements were not being deleted in VoglEditor.
* The ApiCallTreeModel (which maintains all the API calls and snapshopts) was being allocated each time a trace was opened, but never deleted when the trace was closed.
* The StateTreeModel was being reallocated each time the UI was updated to a new snapshot, but they were never being deleted.
* ApiCallItems were being deleted by BOTH the FrameItem and the ApiCallTreeItem; they are now only deleted by the ApiCallTreeItem.