Implement key up/down handling for all platforms

[ghewgill]

From Barry Mead:

Currently your calculator does not implement any KEY-HOLD functionality.
The real HP-15C and most of the simulators out there simulate the KEY-HOLD
functionality of the real calculator. What I mean by KEY-HOLD is that when
you hold down on the key (or mouse button) the display continues to display
the value that is shown when the key is initially pressed. This is very
important for the "Clear-Prefix","Single-Step","Mem", and " (i) " functions.