Parts of the expression can now be removed from the expression

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Drawing the MathObject is now cached by MathView
Fixes #63

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Changed the default size of expressions to a better value for normal screens

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