Fixed some bugs.
- Fixed issue #36
- When the contents of a power were empty one could enter "0" as a power

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
The MathSymbolEditor can now be scrolled in the keyboard
+ Added a scroller for the MathSymbolEditor
+ Added some padding between different symbols
* Did some refactoring

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
The keyboard is now used for entering constants

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