endSessionDialog: Support rebooting into the bootloader menu aka ("Boot Options")

This implements the "Alt" behavior for the "Reboot" button as outlined in
the design here: https://wiki.gnome.org/Design/OS/BootOptions

Note I've tried implemeting this with the AltSwitcher class from
js/ui/status/system.js first, but that puts the button in a St.Bin()
which causes the button to think it is the only button on the dialog
and makes it have rounded corners on both of its bottom corners.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/199

js/ui/endSessionDialog.js

@@ -213,6 +213,11 @@ class EndSessionDialog extends ModalDialog.ModalDialog {
                       destroyOnClose: false });
 
         this._loginManager = LoginManager.getLoginManager();
+        this._loginManager.canRebootToBootLoaderMenu(
+            (canRebootToBootLoaderMenu, unusedNeedsAuth) => {
+                this._canRebootToBootLoaderMenu = canRebootToBootLoaderMenu;
+            });
+
         this._userManager = AccountsService.UserManager.get_default();
         this._user = this._userManager.get_user(GLib.get_user_name());
         this._updatesPermission = null;
@@ -239,6 +244,9 @@ class EndSessionDialog extends ModalDialog.ModalDialog {
         this._totalSecondsToStayOpen = 0;
         this._applications = [];
         this._sessions = [];
+        this._capturedEventId = 0;
+        this._rebootButton = null;
+        this._rebootButtonAlt = null;
 
         this.connect('destroy',
                      this._onDestroy.bind(this));
@@ -363,6 +371,26 @@ class EndSessionDialog extends ModalDialog.ModalDialog {
         this._sessionSection.visible = hasSessions;
     }
 
+    _onCapturedEvent(actor, event) {
+        let altEnabled = false;
+
+        let type = event.type();
+        if (type !== Clutter.EventType.KEY_PRESS && type !== Clutter.EventType.KEY_RELEASE)
+            return Clutter.EVENT_PROPAGATE;
+
+        let key = event.get_key_symbol();
+        if (key !== Clutter.KEY_Alt_L && key !== Clutter.KEY_Alt_R)
+            return Clutter.EVENT_PROPAGATE;
+
+        if (type === Clutter.EventType.KEY_PRESS)
+            altEnabled = true;
+
+        this._rebootButton.visible = !altEnabled;
+        this._rebootButtonAlt.visible = altEnabled;
+
+        return Clutter.EVENT_PROPAGATE;
+    }
+
     _updateButtons() {
         this.clearButtons();
 
@@ -384,8 +412,35 @@ class EndSessionDialog extends ModalDialog.ModalDialog {
                 },
                 label,
             });
+
+            // Add Alt "Boot Options" option to the Reboot button
+            if (this._canRebootToBootLoaderMenu && signal === 'ConfirmedReboot') {
+                this._rebootButton = button;
+                this._rebootButtonAlt = this.addButton({
+                    action: () => {
+                        this.close(true);
+                        let signalId = this.connect('closed', () => {
+                            this.disconnect(signalId);
+                            this._confirmRebootToBootLoaderMenu();
+                        });
+                    },
+                    label: C_('button', 'Boot Options'),
+                });
+                this._rebootButtonAlt.visible = false;
+                this._capturedEventId = global.stage.connect('captured-event',
+                    this._onCapturedEvent.bind(this));
             }
         }
+    }
+
+    _stopAltCapture() {
+        if (this._capturedEventId > 0) {
+            global.stage.disconnect(this._capturedEventId);
+            this._capturedEventId = 0;
+        }
+        this._rebootButton = null;
+        this._rebootButtonAlt = null;
+    }
 
     close(skipSignal) {
         super.close();
@@ -396,14 +451,21 @@ class EndSessionDialog extends ModalDialog.ModalDialog {
 
     cancel() {
         this._stopTimer();
+        this._stopAltCapture();
         this._dbusImpl.emit_signal('Canceled', null);
         this.close();
     }
 
+    _confirmRebootToBootLoaderMenu() {
+        this._loginManager.setRebootToBootLoaderMenu();
+        this._confirm('ConfirmedReboot');
+    }
+
     _confirm(signal) {
         let callback = () => {
             this._fadeOutDialog();
             this._stopTimer();
+            this._stopAltCapture();
             this._dbusImpl.emit_signal(signal, null);
         };