Move the keysym ↔ Unicode table to .rodata

Move the keysym ↔ Unicode table to .rodata

The table we use for converting between keysyms and Unicode should be
static and constified, so that it can live in the .rodata section of
the ELF shared object, and be shared among processes.

This change moves the table to a source file, instead of an header; the
change also requires the clutter_keysym_to_unicode() function to be
moved from clutter-event.c into this new source file. The declaration is
still in clutter-event.h, so we don't need to do anything special.