WIP: Move objects with keyboard

[vozdeckyl]

Implements #2490.

However, it still doesn't work 100%. For some reason, the first move is never recorded and I don't know why.

Steps to reproduce:

1. create an object (rectangle, for example)
2. select the object by clicking on it
3. press Shift+Left once
4. press the undo button
5. the object disappears instead of moving back to its original position
6. now repeat steps 1-4, it works this time!

[vozdeckyl]

The issue was fixed by using a different method. The movement is initiated by a key press and stopped by a key release. In order to achieve this, the shortcuts for moving/resizing the selection had to be removed, and its functionality had to be hard-coded. The drawback here is that the user will no longer be able to use different keys for moving/resizing the selection.

[borgmanJeremy]

Is this ready for review / merging?

[vozdeckyl]

Hi @borgmanJeremy, I think so!