WIP: Win32 rewrite keymap

Note: This MR is old and has been superseded by !1051 (merged)

Get rid of the humongous lookup table. This cuts 600 ms startup latency on my machine. It makes the start of a GTK3 application feel much snappier. See #2055 (closed)

I highly recommend applying !991 (merged) first, since this is intended to be used in conjunction with that. Otherwise expect some glitches relating to dead keys, since I removed some code.