Created a substitution dialog

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Added support for substitutions in the database

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Merge remote-tracking branch 'folkert/work' into work

Conflicts:
	src/org/teaminfty/math_dragon/MainActivity.java
	src/org/teaminfty/math_dragon/model/Database.java

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Fixed a bug that occurred when upgrading the database

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Substitutions can now be retrieved from the database by name

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