Support files without an extension in XDG portal filters

Due to the glob being used ("*.{file_extension}"), there was no
possibility to include files without a dot in their name when using filters.
The glob "*" is now used when an empty filter ("") or a match-all
filter ("*") is specified, which replaces globs "*." and "*.*" respectively.
The glob "*." did not make sense, as it matched files ending with a dot,
and "*.*" did not match files without an extension (which are quite popular on Linux).

src/backend/xdg_desktop_portal.rs

@@ -28,7 +28,11 @@ impl From<&Filter> for FileFilter {
     fn from(filter: &Filter) -> Self {
         let mut ashpd_filter = FileFilter::new(&filter.name);
         for file_extension in &filter.extensions {
-            ashpd_filter = ashpd_filter.glob(&format!("*.{file_extension}"));
+            if file_extension == "*" || file_extension == "" {
+                ashpd_filter = ashpd_filter.glob("*");
+            } else {
+                ashpd_filter = ashpd_filter.glob(&format!("*.{file_extension}"));
+            }
         }
         ashpd_filter
     }