Variable buttons are now disabled for the substitution editor.
Fixes #96

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Fixed the layout for the substitution editor dialog for tablets.
Also made the operation drawer a bit smaller.

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Integrals and derivatives now standard have an 'x' filled in

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