C. Menu label positions, selection logic additions

* The positions of the circle menu labels are now positioned at the
center of mass of each part, instead of the center of its boundaries.
This should hopefully result in better positioning for certain option
counts/sizes.
* Two things were changed regarding item selection (they replicates
Windows' behaviour):
- When items are selected and an empty area is clicked with CTRL down,
nothing happens - CTRL must be released for deselecting.
- When an unselected item is right-clicked, but unselected yet, it will
turn selected.