Fix #38, set a maximum size for the MathObjects

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Parentheses can now be saved/loaded to/from XML

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Fixes #42, where the evaluation screen was too small

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Parentheses are now correclty placed around MathSymbol if it's the base of MathOperationPower

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