Fix bug: prevent click on x/y button...

...when answer contains the word feet or inches
Equation.equals() now compares results as well

This is so that if the equation is the same but the answers are
different (because of order of operations), both will show up in the
recent history.
Ft and In now replace eachother (like operators do)

For example: if you press 1' and then change your mind you can just
press inches instead of hitting backspace