Constants for default window size.
Update premake4.lua
Update snow-common include paths.
Use utf8.hh from snow-common.
Lexer stuff.

Unreachable code: gone.
Pointless code: gone.
Temporarily remove find_invalid call.

No equivalent call available in snow-common's utf8.hh. Might write
something similar.
Use string, not string_t.

Old type was string_t.