Fixed the absolute paths in the desktop file

The application icons now install into the system icon theme. This also
results in the system always picking the best available icon to display.

To make installing simpler (and to make the names more consistent) the
icon files were moved into the folders of their resolution.

Fixes #383

(cherry picked from commit 9aeac457bb7f98f5d3987164a74b8445dd9977c7)