Switch to PyXDG.IconTheme for icons lookup

[pinotree]

Use PyXDG.IconTheme to lookup the application icons instead of pyGTK; this has some advantages:

• we are already using PyXDG, so it does not add new requirements
• the icon lookup follows the icon theme specs a little bit closer
• the icon lookup does not require an X/graphical session
• there is no implicit adwaita or gnome icon theme used in lookup

there are also few disadvantages:

• there is no concept of "default icon theme", so hicolor is the default theme; a new --icon-theme option is added to select which theme to use
• it seems the lookup logic for hicolor returns the smallest icon available in case the current size is not available: for example a 16px PNG is returned instead of a 48px for a requested and not found 64px size

[gapan]

I'm sorry, but detecting the icon theme is very important functionality that should not be removed. Having to specify the icon theme is too cumbersome, I don't want to do that.

[pinotree]

The problem I see is that the "current icon theme" in reality is the GTK icon theme, which follows the GTK configuration. There is nothing that says it is the same for non-GTK applications.

[gapan]

That's not a problem in my opinion. In my experience at least, most people that use a WM such as the ones supported by xdgmenumaker, mostly use GTK apps.

[donmor]

Fetching current icon theme is not so hard to achieve: gsettings get org.gnome.desktop.interface icon-theme
There may be better ways to fetch it. Using the fetched icon theme as a default should be a solution.

[donmor]

BTW, is it possible to add the --icon-theme option using pyGTK? Also, there could be multiple icon themes specified, to make up a chain of fallbacks.