weston.ini: Introduce core section and remove type keys.

Since a6813d288, there is no longer a "type" key in the [shell] config section.
Instead the code accepts a [core] section with a modules key, containing a
comma separated list of modules to load. This patch removes the type keys
and adds a core section in the example weston.ini config file.

weston.ini

@@ -1,5 +1,7 @@
+[core]
+#modules=desktop-shell.so,xwayland.so
+
 [shell]
-type=desktop-shell.so
 background-image=/usr/share/backgrounds/gnome/Aqua.jpg
 background-color=0xff002244
 panel-color=0x90ff0000
@@ -8,7 +10,6 @@ animation=zoom
 #binding-modifier=ctrl
 #num-workspaces=6
 
-#type=tablet-shell.so
 #lockscreen-icon=/usr/share/icons/gnome/256x256/actions/lock.png
 #lockscreen=/usr/share/backgrounds/gnome/Garden.jpg
 #homescreen=/usr/share/backgrounds/gnome/Blinds.jpg