Test signal emission on selection changes

Test signal emission on selection changes

This commit adds a unit test that changes the selection in various
ways, verifies the result and checks that the selection manager's
selectionChanged signal has been emitted correctly. The test is
data-driven, so I hope that most further testing needs can be
fulfilled by adding new test data.

Moreover, I changed selectedItems() such that the anchored
selection is only taken into account if anchor and current item
are different. The reason is that in some situation the anchor
should not be selected initially (i.e., if an already selected
item is Control-clicked). If the anchor should be selected from
the beginning, it must be selected manually.