keyboard: Try harder to find a matching layout

While we support a reasonable list of layouts nowadays, we don't
include many variants like `fr+oss`. Instead of directly falling
back to the `us` layout, try stripping the variant first, as the
base layout is likely closer to the expectation than `us`.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1907


(cherry picked from commit c1ec7b2ffa)

js/ui/keyboard.js

@@ -466,10 +466,17 @@ Signals.addSignalMethods(Key.prototype);
 
 var KeyboardModel = class {
     constructor(groupName) {
+        let names = [groupName];
+        if (names.includes('+'))
+            names.push(groupName.replace(/\+.*/, ''));
+        names.push('us');
+
+        for (let i = 0; i < names.length; i++) {
             try {
-            this._model = this._loadModel(groupName);
+                this._model = this._loadModel(names[i]);
+                break;
             } catch (e) {
-            this._model = this._loadModel('us');
+            }
         }
     }