Disable default image background, show desktop

[Azaziah]

At the moment OpenLP displays image bg and bg colour
after startup on the primary screen.
This image + bg combo hides any other content until it is closed.

This is bad because:
a) It hides everything the user has running
b) It will act as blank to black and hide everything until
   it is closed via OpenLP, this might be confusing
c) All users don’t want to display logo on
   screen on startup and feel it disturbing
d) If OpenLP crashes, the logo will be shown during the service

Suggestion:
Add checkbox “Don’t use default background”
^^ This would show desktop while not projecting
   anything to the primary screen.

Escape item hides the default image background and shows desktop
but does not set any blank to… active so this might be helpful.