theme: Rename _app-switcher.scss to _switcher-popup.scss

Since the main classes in that file are not dedicated to the app
switcher, rename it to something more appropriate.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/938

data/theme/gnome-shell-sass/_widgets.scss

@@ -27,7 +27,7 @@
 @import 'widgets/network-dialog';
 // OSDs
 @import 'widgets/osd';
-@import 'widgets/app-switcher';
+@import 'widgets/switcher-popup';
 @import 'widgets/workspace-switcher';
 // Panel
 @import 'widgets/panel';