grabHelper: Add (promised-based) grabAsync()

Some GrabHelper uses are in the form:

    doPreGrabStuff();

    this._grabHelper.grab({
        onUngrab: () => {
            undoPreGrabStuff();
        },
    });

A promise-based variant allows to write this more cleanly as:

    doPreGrabStuff();

    await this._grabHelper.grabAsync();

    undoPreGrabStuff();

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/903

js/ui/grabHelper.js

@@ -194,6 +194,15 @@ var GrabHelper = class GrabHelper {
         return true;
     }
 
+    grabAsync(params) {
+        return new Promise((resolve, reject) => {
+            params.onUngrab = resolve;
+
+            if (!this.grab(params))
+                reject(new Error('Grab failed'));
+        });
+    }
+
     _takeModalGrab() {
         let firstGrab = this._modalCount == 0;
         if (firstGrab) {