Using AsyncTask for backspace hold--not working reliably
-Added red color for hold animation, finishes turning red long before clear operation happens
-If you release button after it's red, clear will happen a few hundred ms later...
Fully implemented hold to red color, not the cleanest code though
-Used runnable and handles, but color seems to change too fast and
generally could look better (probably should use animations)
Changed the color scheme
Added smart insertions into expression, auto close ")"'s etc
Fixed some cursor insertion bugs