Drag and drop support

It is now possible to place object sprites on the editor canvas by
dragging them in.
Furthermore, an object can be moved after being placed inside the canvas
by simple drag and drop.

Note about the previous commit: SAVE/LOAD features were added, JSON
format's been preferred over XML.
Merge branch 'master' of https://github.com/RumRogers/AdvV-Refactor