Perform a slight fade-in when showing cinnamon settings.  The reason for this
is, when calling a panel directly (i.e. cinnamon-settings sound) you can sometimes
see the full window/icon view first before it switches to the selected panel
and the window resizes.  Unfortunately, to calculate the resizes properly, we have
to 'show' the window so all its children are sized up and realized.

This is a bit of a trick, starting the window fully transparent, and only
turning opaque once everything is loaded.