setup.py: fix icon path for user installation

[toxeus]

Before, the user installation used the wrong directory
to store the application's icon resulting in the desktop
environment falling back to an ugly default icon. Now,
the correct icon is displayed in the menu.

"By default, apps should look in $HOME/.icons (for backwards
compatibility), in $XDG_DATA_DIRS/icons and in /usr/share/pixmaps
(in that order)."
https://specifications.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html

Contributed by FDF

[bauerj]

That sounds right but doesn't your commit put it in ~/.local/share/icons if $XDG_DATA_HOME is not set? According to your quote it should look in ~/.icons in stead.

[toxeus]

Nope. ~/.icons should not be used by new apps that's why they explicitly mention it as backwards compatibility. ~/.local/share is the XDG spec's fallback if $XDG_DATA_HOME is not set.

spec:

$XDG_DATA_HOME defines the base directory relative to which user specific data files should be stored. If $XDG_DATA_HOME is either not set or empty, a default equal to $HOME/.local/share should be used.