[horhulenko]

ModuleModel had no destructor, so when one was destroyed, its root item and every item under it stayed in memory. On master, this happens every time the content of a grouping is shown and closed.

The fix deletes the root item in ~ModuleModel, the same way GatePinsTreeModel and ModulePinsTreeModel already do. It can't go into BaseTreeModel: ModuleItem removes itself from the model's maps when it's deleted, and those maps would already be gone by then.

Memory left after showing and closing the content of 50 groupings of 1000 gates, 4 times in a row:

	round 1	round 2	round 3	round 4
before	+10.7 MB	+21.4 MB	+32.1 MB	+42.7 MB
after	+0 MB	+0 MB	+0 MB	+0 MB