Select items (and execute batch actions)

* You can now select desktop items by clicking on them. This deselects
the previous element(s), unless holding down CTRL, which allows for
multi-selection (just as in most operating systems by default). Deselect
all by clicking on an empty area.
- Launching an item now requires a double-click.
- When opening the circle menu with multiple items selected, it opens in
the center of the selection.
- The actions in the circle menu will be executed for every selected
item.