Start the game in fullscreen mode based on the player’s configuration

We save the on-or-off fullscreen switch in the player’s configuration
file but we never use it when loading the game.  This patch changes
the initial start-up call to love.graphics.setMode() so that it passes
on the third and optional argument: a boolean indicating whether or
not to use fullscreen mode.

The value we give to that function is new: Settings.FULLSCREEN.  We
define it in the settings module based on the player’s configuration.
If a player changes the fullscreen setting while player then the value
of Settings.FULLSCREEN **does not** receive the new value.  When going
to and from fullscreen mode in the options menu we directly call
love.graphics.setMode().  And there we use the boolean we get from the
checkbox, not the one from the Settings module.  The only time we use
the Settings.FULLSCREEN boolean is when we first start the game.
After that any switches in-or-out of fullscreen mode do not need to
worry about updating the Settings.FULLSCREEN value.  All that said, we
do update the fullscreen property of the player’s configuration
based on what he or she does in the options menu.