tools: add xkbcli-scaffold-new-layout as helper tool

tools: add xkbcli-scaffold-new-layout as helper tool

This tool set ups the required directory structure and template files to add new
keyboard layouts or options. For example, run like this:

    xkbcli-scaffold-new-layout  --layout 'us(myvariant)' --option 'custom:foo'

This will up the evdev rules file, the evdev.xml file, the symbols/us file and
symbols/custom file in $XDG_CONFIG_HOME so that the user has everything in place
and can start filling in the actual key mappings.

This tool is currently uninstalled until we figure out whether it's useful.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>