Pluginsystem

Rebased:
Completely rewirtes EntityProps, makes them able to be edited in the IDE.
Adds Plugins.

Also includes #564 -> Fixes #563. Mac menu option still has to be added!

![plugineditor](https://cloud.githubusercontent.com/assets/5982050/4108739/e76177e2-31db-11e4-8870-aa5b94236753.PNG)
![sheetineditor](https://cloud.githubusercontent.com/assets/5982050/4108738/e7601bb8-31db-11e4-89b0-7f0dcd08de75.PNG)