list view: Add pressed() and clicked() signals

[peter-ha]

Resolves: #515

[peter-ha]

• pressed() only emitted with mousePress, not with key navigation
• pressed() makes sense in addition to selectedRowChanged(), because it will
  be emitted also in the case of NoSelection, and emitted in case the row
  did not change
• clicked() is also emitted when NoSelection is the current selection mode
• clicked() is only emitted if the pressed row is the same as the released one