Scrollable desktop, item right-click fix

* The Jesk desktop is now scrollable. You can navigate around on it by
moving your mouse while holding down the middle mouse button (the
wheel), or by moving the cursor to any of the four screen edges. Items
are freely positionable everywhere.
* Previously, when an unselected item was right-clicked, it was added to
the current selection before opening the circle menu. This was thought
to resemble the Windows explorer behavior, while it actually didn't. The
selection will now switch over to the clicked item if it was not
selected yet, instead of adding it.

Developers:
* A few changes to the positioning and dragging logic. Especially
important:
- getX() and getY() are now meant to be relative to the parent object
[getParent()], while getXOnScreen() and its Y counterpart are the real
coordinates.