Tests and related bugfixes for BaseStore.remove

[gycsaba96]

I wrote unit tests for BaseStore.remove and discovered 2 bugs along the way:

• When removing an item, the code only considered the children instead of removing all descendants. This resulted in the store being left in an inconsistent state. (This seems related to the scattered use of BaseStore.refresh_lookup_cache method.)
• When removing a tree of items, only the root element was reported with a removed signal. (If I am not mistaken, the code never relied on this signal, but let's fix it anyway.)

The main changes are as follows:

• write unit tests for BaseStore.remove,
• ensure that all descendants are recursively removed,
• ensure that a removed signal is emitted for each removed item,
• make SavedSearch a subclass of StoreItem to keep things readable.

[diegogangl]

LGTM, thanks!