The way of icon path lookup for `menubar` is enhanced so that it is
based on a theme-oriented way as described in the specification:
Icon Theme Specification, Ver. 0.12
http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html
To accomplish this:
* Add the two new files `icon_theme.lua` and `index_theme.lua`.
The former implements an icon lookup algorithm suggested in the URL
above. The latter implements a helper object to parse the cache file
`index.theme` of which data is used by the former.
* Modify `menu_gen.lua` to use the new algorithm.
- The implementation of `lookup_category_icons` is changed
accordingly.
- The values of the field `all_categories.icon_name` are changed file
names to icon names, i.e., file extensions which are used to
indicate image file formats are removed.
* Add the new file `icon_theme_spec.lua` for a unit test for checking
if `icon_theme.lua` together with `index_theme.lua` works as
expected.
Daniel Hahler
Kazunobu Kuriyama