(see Desc) Very Minor Tweaks

* EnemyType renamed to ShipType
* Fixed RedShip and WhiteShip being swapped
* Some classes now contain forward declaration
* Player Speed increased
(see Desc) Added Commands!

Command-Related Things:
* Added SceneNode::getCategory()
* Added SceneNode::onCommand()
* Added namespace/enum "Category" to distinguish between similar
SceneNodes
* Added Struct "Command" for issuing commands to all SceneNodes of a
specified Category
* Added Class "CommandQueue" for issuing multiple commands in succession
* World instance now contains a CommandQueue and a function
getCommandQueue()

Unrelated Things:
* Function Definitions for SceneNode::attachChild() and
SceneNode::detachChild() were shifted downward
* Reworded SceneNode::updateChildren()'s for loop