Add command for target self.

Move autopilot to Target + Y.  Assign target self to Target + Left stick
click.
Fix up briefing screen for gamepad.

Return to main menu on B.  Show object data on UP and DOWN.
Transparent background for gamepad images.
Remove doc/gamepad.rst.

It'll go on arescentral.org instead.
Update doc/img/target.xml.

Move Autopilot to Y, change LSB to Target Self.
Merge branch 'gamepad' into develop