CcDefaultAppsRow: avoid "notify::selected-item" initial firing

When connecting to notify::selected-item signal of a CcDefaultAppsRow widget, an unsolicited initial firing of that signal happens just after the widget is constructed.

To prevent that, so we can avoid the undesired consequences of it described in #2626 (closed) and #2683 (closed), we connect it in an idle handler, so at that time the widget has already been constructed and the initial firing already took place when no signal handler was connected.

Fixes #2626 (closed)
Fixes #2683 (closed)