File saving and loading doesn't work on Linux with XDG portals disabled

[YuriSizov]

When you try to save, load, import, or export files in Godot, nothing happens. In the logs you can see this issue:

ERROR: Failed to send DBus message: Could not activate remote peer 'org.freedesktop.portal.Desktop': activation request failed: unit is masked
   at: file_dialog_show (platform/linuxbsd/freedesktop_portal_desktop.cpp:460)

This can happen if the XDG portals feature is not supported (file chooser portal specifically), or if the feature is masked out. The issue for Bosca is that the engine doesn't handle this properly and doesn't report this properly. Ideally, the engine should check if portals are enabled and disable the native file dialog feature if not, falling back to the built-in dialog. There is already a check for the FEATURE_NATIVE_DIALOG_FILE feature flag in place, but on both X11 and Wayland it simply returns true, always.

We probably cannot detect the issue, although some trickery may be possible (but would require testing on affected platforms). So a possible solution could be to provide an option for users to disable native dialogs entirely. Either way, if we cannot use native dialogs, the built-in dialog needs to have its visuals cleaned up to fit Bosca's look.

[YuriSizov]

It looks like we're getting this situation addressed in the engine with godotengine/godot#101812! Remains to be seen if it's going to be available in Godot 4.3.x at some point, because I don't have plans to update to 4.4+ at this time. In any case, we'd still have to clean up the visuals of the built-in file dialog to make use of this.