Implemented the basis of the interface
+ Created a fragment from where the operation can be dragged
+ Created a fragment which contains the main screen
+ Created a drawer layout
+ Added resources (our color palette and some images needed for the drawer layout)
- Removed some unnecessary auto-generated code

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Started on the basis of the logic that draws (and measures) the formulas which are stored in MathObjects

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Integrated the engine with the interface class structure.
From now on we'll be using Symja for the mathematical calculations.

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Discovered that the application package was still set to com.example.math_dragon.
Changed it to the right one (org.teaminfty.math_dragon).

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Subtract methode plus een methode voor het checken van lege childs