helpers: Use native Python to iterate $PATH
helpers: Use native Python to iterate $PATH

Using ls and tr means that we have two external commands just to iterate
through the $PATH environment variable.

This not only is kinda pointless to do (we *can* do this in Python
without forking off) but is also error-prone because it relies not only
on ls/tr being there but also on their outputs being correct.

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>

karlch/vimiv,karlch/vimiv,karlch/vimiv