Add "initial-bullets" setting

	* data/org.gnome.gspaceduel.gschema.xml: Add "initial-bullets" uint field.
	* data/preferences.ui: Allow setting "initial-bullets".
	* src/application.vala: Bind setting to widget.
	* src/game.vala: Set intial amount of bullets for each ship.
	* src/sprites.vala: Add ShipSprite::has_bullets() mehtod.