Improve machine API to the themes

* Improve index/index.json
* Add MD5 sum of that file into index/index.json.md5
* Remove experimental index.conf and index.xml files from the build
* Remove genindex script and related template files
* Add screenshot-missing.png icon to screenshots dir
  - From elementary-xfce icon theme (status/64/image-missing.png)
* Update index/README.md detailing the format/fields of the JSON file.
Change THEMES_VERSION to match next Geany version

* Remove naive/stupid version mismatch tests on Geany version
* Reformat indentation a bit in Makefile
Add `make help` to Makefile with a little bit of info
Add simple `install.sh` script

In case someone is too lazy to copy manually and also to lazy to
install and use GNU Make.
Switch version number scheme used in themes

From now on the version field in color scheme files is a simple
integer incremented each time the scheme is changed. To avoid
arbitrarily re-setting all versions to 0 (or some other number), I
just removed the decimal point from the existing versions so they
still have some significance.

* Update mkindex.py to handle new format
* Remove deleted note from index/README.md
* Update index/index.json and MD5 sum of it
Add automatic version bumping script

Makefile default rule now calls `autobump` and `index` targets. To
use this, call `make` after changing any of the colour schemes.

The way it works is versions.log contains the md5 sums of the
color scheme files. The autobump script checks the old sum from
versions.log against the current color scheme file and if any of
them changed, it opens the color scheme and increments the version
number in it and then stores the newly computed sum (after the bump)
back into versions.log. When the `index` rule is run in the Makefile,
it updates the `index.json` and `index.json.md5` files based on the
changes.
Update comment in color schemes about version bumping

This is also a good way to test/demo the new script :)