[xkb] `*.xkb_keymap` and `*.kkb_symbols` outputs

[fabi1cazenave]

fixes #97 and outputs less ambiguous xkb files (hopefully):

• the .xkb_keymap file (formerly *.xkb) is a full, standalone file, to be used with xkbcomp
• the .xkb_symbols file (formerly *.xkb_custom) is meant to be put in xkb/symbols/custom

Both files now have a header that should give clear explanations on their purpose.