XDG paths are hardcoded

Right now, LocateGames in ModdableGameLocator.cs hardcodes XDG paths:

https://github.com/hedge-dev/HedgeModManager/blob/3a1c8652a9aedce13e19fe8e335416d8389b7cc7/Source/HedgeModManager/ModdableGameLocator.cs#L290-L299

All of these paths (~/.config, ~/.local/share, etc.) are configurable and have multiple possible values. In my case, HMM couldn't find UnleashedRecomp because it was installed in a different directory in $XDG_DATA_DIRS. (home-manager installed it at /etc/profiles/per-user/quentin/share/applications/UnleashedRecomp.desktop.)

https://specifications.freedesktop.org/basedir-spec/latest/#variables contains the proper resolution logic for each directory; in summary the rules are:

• Config should be written to $XDG_CONFIG_HOME, or if that is unset, $HOME/.config. ($XDG_CONFIG_DIRS should be searched for read-only config, but I don't think you have any of that?)
• Applications should be searched for in $XDG_DATA_HOME, or if that is unset, $HOME/.local/share, and then in $XDG_DATA_DIRS, or if that is unset, /usr/local/share/:/usr/share/

In addition, you might want to search for the application by MIME type (x-scheme-handler/unleashedrecomp) instead of searching by .desktop file name, since different packages are using different names for the .desktop file. Though this would overlap with the Flatpak detection; I'm not sure how you might want to handle that.

With the current rules, globally-installed UnleashedRecomp such as the AUR package won't be found.

Start in Source/HedgeModManager/ModdableGameLocator.cs around LocateGames at lines 290-299, then read the linked freedesktop.org Base Directory Specification. Trace how XDG_CONFIG_HOME, XDG_DATA_HOME, and XDG_DATA_DIRS should affect application discovery, including the differing .desktop filenames and MIME-type suggestion. Done means globally installed applications such as UnleashedRecomp can be found in configured XDG locations.