shell: Create panel views after desktop views are ready

With the current code, the splash screen will be hidden as soon as the wallpapers are loaded.

However, the splash screnn is actually notified about the desktop stage about 1-1.5 second later after the wallpaper plugin resets the loading property.

The reason for that is that the panel is loaded between wallpaper.loading = false in the wallpaper and ShellCorona::checkAllDesktopsUiReady().

This change re-arranges the startup sequence so the panels are loaded after the desktop views become ready. It reduces plasma startup time a bit.

In long term, we should look for making panel loading as async as possible so the main thread doesn't get blocked for too long.