appDisplay: Move child focus handling to IconGrid

The icon grid is always paginated, so the app grid code doesn't need
to behave differently in the FolderView and AppDisplay.

Move the keyboard handling to IconGrid itself, and remove the now dead
code from AppDisplay.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1271

js/ui/iconGrid.js

@@ -1143,7 +1143,6 @@ var IconGrid = GObject.registerClass({
     Signals: {
         'pages-changed': {},
         'animation-done': {},
-        'child-focused': { param_types: [Clutter.Actor.$gtype] },
     },
 }, class IconGrid extends St.Viewport {
     _init(layoutParams = {}) {
@@ -1199,7 +1198,9 @@ var IconGrid = GObject.registerClass({
     }
 
     _childAdded(grid, child) {
-        child._iconGridKeyFocusInId = child.connect('key-focus-in', actor => this.emit('child-focused', actor));
+        child._iconGridKeyFocusInId = child.connect('key-focus-in', () => {
+            this._ensureItemIsVisible(child);
+        });
 
         child._paintVisible = child.opacity > 0;
         child._opacityChangedId = child.connect('notify::opacity', () => {
@@ -1210,6 +1211,14 @@ var IconGrid = GObject.registerClass({
         });
     }
 
+    _ensureItemIsVisible(item) {
+        if (!this.contains(item))
+            throw new Error(`${item} is not a child of IconGrid`);
+
+        const itemPage = this.layout_manager.getItemPage(item);
+        this.goToPage(itemPage);
+    }
+
     _setGridMode(modeIndex) {
         if (this._currentMode === modeIndex)
             return;