baseAppView: Call loadGrid() directly

baseAppView: Call loadGrid() directly

Now that the three views follow the exact same loading routine
(remove all + load apps + load grid), we don't need each view
call loadGrid() directly anymore.

This is an important step in order to animate adding and removing
icons, since now we can diff old and new app icons properly.

Move all calls to BaseAppView.loadGrid() to a single one after
BaseAppView._loadApps(). Also add the underscore prefix, since
this is now considered a protected function.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/645