Created a class MathObjectEmpty to represent empty children

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
The drag state is now set correctly to all children of the MathObject that's being dragged
Also a deep copy of the MathObject is now created for dragging

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Implemented the drag-and-drop system

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Implemented MathParentheses

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Parentheses are now placed when dragging to the screen

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Power operations can now also be dragged from the drawer

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Merge branch 'master' of https://github.com/Divendo/math-dragon

Conflicts:
	src/org/teaminfty/math_dragon/FragmentMainScreen.java