notification icon changed to black/white
Merge pull request #1007 from Catrobat/cat-943

CAT-943
notification icon changed to black/white
Introduced ConcurrentFormulaHashMap to BrickBaseType
Introduced custom xstream converter to support porting old projects' code.xml to new structure of code.xml
Converted FormulaBrick.java from interface to a class
Converted NestingBrick.java from abstract to interface
All the DroneBricks are extended by BrickBaseType.java