js/appDisplay: Generalize app scrollview CSS

We want to show left/right side pages during navigation, also in
FolderViews. Let this scrollview use the same style than the "all
apps" one, and generalize the name a bit.

This will compress the scrollview horizontally, so there's actual
overflow space to show these pages.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1630>

data/theme/gnome-shell-sass/widgets/_app-grid.scss

@@ -124,7 +124,7 @@ $app_grid_fg_color: #fff;
 }
 
 // Some hacks I don't even know
-.all-apps {
+.apps-scroll-view {
   // horizontal padding to make sure scrollbars or dash don't overlap content
   padding: 0 88px;
 }

js/ui/appDisplay.js

@@ -137,6 +137,7 @@ var BaseAppView = GObject.registerClass({
 
         // Scroll View
         this._scrollView = new St.ScrollView({
+            style_class: 'apps-scroll-view',
             clip_to_allocation: true,
             x_expand: true,
             y_expand: true,
@@ -968,8 +969,6 @@ class AppDisplay extends BaseAppView {
         this._pageManager = new PageManager();
         this._pageManager.connect('layout-changed', () => this._redisplay());
 
-        this._scrollView.add_style_class_name('all-apps');
-
         this._stack = new St.Widget({
             layout_manager: new Clutter.BinLayout(),
             x_expand: true,