Touchscreen is now required, fixes #75

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
The evaluate buttons are now disabled when an expression isn't completed.
Fixes #74

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Added an operations source layout for landscape mode for normale screens.
Fixes #71

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