IconCache: get the proper theme path based on the fact we're using a themed icon or not
If an app uses QIcon::fromTheme then we only have the icon name of it, and thus we need to return a valid icon theme path. Now, it's not possible to return the whole list of dirs, so we just fallback to the home icons older, so that users might in case add icons there.
In $SNAP world, when using desktop-launcher ~/.local/share/icons also contains all the themes in from /usr/share/icons and potentially more (depending on the wrapper used for launching) so, we can be quite flexible with it.
Also when running in a snap, save icons in $XDG_CACHE_HOME or in the user cache, but still in an indicator readable location (LP: #1600136)