Prefab Support

[Felix Rohrbach]

Hi there,

Based on the work of Leinir, I've worked a bit on the prefab support of gluon 
and I think I've reached a state where it can be merged back into master. 
Current version is in the branch prefab.

What should work now:
- Creating prefabs from GameObjects by drag'n'drop
- Creating instances of prefabs by drag'n'drop
- Reverting and applying changes of a prefab

What does not work:
- Reverting/applying only one child of a prefab instance. There is a button, 
but it doesn't do anything. It can be removed for a release.

What was not tested:
- Prefabs in prefabs

The branch has 7 commits. The diff between the master and the prefab branch is 
1624 lines (887 additions, 275 removals).

Do you want to review it on reviewboard, or is that too much work for you?

Regards,
Felix