Clear the selection in MousePressEvent() rather than MouseReleaseEvent()

This fixes the problem that clicking an unselected item in order to drag
it would result in dragging all previously selected items as well. With
this commit, previously selected items are unselected when a new item is
clicked.

The reason why clearing the selection was moved to MouseReleaseEvent()
in commit b583dd6d was that clicking one
of several selected items should not result in unselecting the other
items (to make sure that dragging multiple items is possible). However,
this can also be assured by just checking in MousePressEvent() if the
clicked item has been selected already and not clearing the previous
selection in that case. This applies equally to the case that a context
menu is requested when several items are selected.