Add experimental field to list supported Geany versions

This can be used by a colour scheme manager or by the user directly
to see which Geany versions the scheme is compatible with.
Add "screen_hash" field and rename "md5hash" to "scheme_hash"

These are the fields containing an MD5 sum of the screenshot and
color scheme files in the index.
Add "compat" field to index.json file
Allow regular version number strings (trailing .0 not required)

The mkindex.py script will add the implicit .0 at the end of the
version triplet where needed.