keepass: improvements

`*.desktop` file now:
 -  Refers to an icon.
 -  Is placed in the proper category (based on comparison with `keepassx2`.
 -  Has proper mime type (also based on comparison).

Also, now use `icoutils` to extract icons from the application ressouces
and transform them from `*.ico` to `*.png`. Created and used a generic
script that has the ability to move the extracted `*.png` to their
appropriate standard freedesktop location.

Tested this on nixos. `keepass` now has a icon and is categorized
in the same bin as `keepassx2`. The program still execute and
function prefectly.