Fix keyboard input

[floooh]

There are several issues with keyboard input:

• the keys =-; don't work for Z9001 in the browser (is this a general 32-bit issue, or just in browser, and are there differences between browsers? -> it looks like the key_mask is deleted too early)
• the last_ascii and 'AnyKeyPressed' hack in Main.cc, this leads for instance to key-repeat when Shift is keeping pressed, but was necessary to make games work
• sometimes the key repeat gets stuck without pressing any keyboard keys
• the yakc class needs a proper key-input method, the current method up in Main.cc is hacky and must be repeated in every other project which uses the emulator

I think a clean rewrite of the keyboard input code would make sense.

[floooh]

Interesting observation: in PNaCl, the keys in question (-,=,;) all produce a warning 'unhandled key code' with the key codes 186, 187 and 189, and PNaCl has the same problem in the Z9001 emu as emscripten (but only the Z9001 emu in emscripten and PNaCl has a problem with those 3 keys).

[floooh]

...it works on Windows compiled as 32-bit, so it's not a 64- vs 32-bit problem.

[floooh]

Fixed.