appDisplay: Connect to hidden signal after content initialization

appDisplay: Connect to hidden signal after content initialization

When the overview is hidden just after starting GNOME Shell the app
grid is not populated yet. Thus, this._grid.nPages is 0. When the
overview is about to be hidden, we call goToPage(0) on the AppGrid.
This results in pageNumber being clamped to -1 and leads to the
following exception:

```
JS ERROR: Exception in callback for signal: hidden: Error: Page -2 does not exist at IconGrid
getItemsAtPage@resource:///org/gnome/shell/ui/iconGrid.js:1383:19
_translatePreviousPageIcons@resource:///org/gnome/shell/ui/appDisplay.js:323:34
_syncPageIndicators@resource:///org/gnome/shell/ui/appDisplay.js:396:14
goToPage@resource:///org/gnome/shell/ui/appDisplay.js:445:14
goToPage@resource:///org/gnome/shell/ui/appDisplay.js:1237:29
goToPage@resource:///org/gnome/shell/ui/appDisplay.js:1584:15
_init/<@resource:///org/gnome/shell/ui/appDisplay.js:1328:52
_emit@resource:///org/gnome/gjs/modules/core/_signals.js:89:42
_hideDone@resource:///org/gnome/shell/ui/overview.js:589:18
_animateNotVisible/<@resource:///org/gnome/shell/ui/overview.js:566:55
onStopped@resource:///org/gnome/shell/ui/overviewControls.js:753:21
_makeEaseCallback/<@resource:///org/gnome/shell/ui/environment.js:150:22
_easeActorProperty/<@resource:///org/gnome/shell/ui/environment.js:316:60
```

Connect to the overview hidden signal only after initializing the
AppDisplay content, so that goToPage(0) is not called when there
is no page yet.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5837
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2536>