Context menu; not click-through anymore (fix)

* A context menu was added. It appears when clicking on a desktop item
and allows opening, editing and printing of that item. This will later
be replaced with a more advanced menu.
* Due to weird behaviour (at least on Windows), the main window (the one
with the icons) only received input on non-transparent locations. This
commit implements a workaround by adding 0.4% of black (the smallest
possible value) to the background.