Now has working screen. All button callbacks work except plus/minus. Note: if there are not enough operands on the stack to complete an operation, the program crashes -- we should probably fix this.Also, it might be nice to divide our screen into two: one to show the top of the stack, and one to show the working number that has not yet been pushed onto the stack. This would require a bit of reorganization of the screen and some reworking of how the string is built up, but I think it'd be worth it.