screenshot-ui: Put screencasts into a subdirectory

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2103>

js/ui/screenshot.js

@@ -1888,11 +1888,16 @@ var ScreenshotUI = GObject.registerClass({
             }
 
             method(
+                GLib.build_filenamev([
+                    /* Translators: this is the folder where recorded
+                       screencasts are stored. */
+                    _('Screencasts'),
                     /* Translators: this is a filename used for screencast
                      * recording, where "%d" and "%t" date and time, e.g.
                      * "Screencast from 07-17-2013 10:00:46 PM.webm" */
                     /* xgettext:no-c-format */
                     _('Screencast from %d %t.webm'),
+                ]),
                 { 'draw-cursor': new GLib.Variant('b', drawCursor) },
                 ([success, path], error) => {
                     if (error !== null) {