shell: Add a new weston.ini to control the startup animation

The new key startup-animation in the [shell] section lets you
control the startup animation.  Default is fade, but with this patch
we can also do none, which just show the desktop as fast as possible.
diff --git a/weston.ini b/weston.ini
index 9a7137f..7006931 100644
--- a/weston.ini
+++ b/weston.ini
@@ -9,6 +9,7 @@
 panel-color=0x90ff0000
 locking=true
 animation=zoom
+startup-animation=fade
 #binding-modifier=ctrl
 #num-workspaces=6
 #cursor-theme=whiteglass