Implement step controls

Mobs move in a grid step fashion like in Pokemon games. Mobs lock into
grid spaces as they move. Implementation mimics that of the Quintus
Javascript game engine library.

Movement does not account for collisions.

Player's MobMover struct altered accordingly.

Movement parameters are as of this commit inflexible.
Implement collision handling

Base class SceneNode has collision handling. Differs from original SFML
project from which this project is based by deferring collision handling
to nodes themselves (instead of being handled by World/Overworld). Works
similarly to update(): derived classes override resolveCurrent() to define
unique class behavior.

Similar to original project in that resolveSceneCollisions() compares one
scene graph to another graph (or itself). Every node of one is compared
with each node of the other. It's also possible to call
resolveNodeCollisions() to compare just the passed node parameter with the
graph calling the method.

Not a particularly efficient implementation compared to one with
quadtrees, but it's a start.
Override resolveCurrent() for Mob

Mobs resolve their collisions uniquely. If it's the mob's first step and
collides with something, it's position is reset to the original. If
however it is not the mob's first step and collides, then the mob
continues to move. This implies that if the mob successfully navigates its
first step, it has the privilege of completing its move even if another
mob attempts to access the same grid location.
Call resolveSceneCollisions() in Overworld

Overworld::update() calls the scene graph's resolveSceneCollisions()
method, passing the scene graph itself as the parameter. This will check
the scene graph's nodes with themselves exhaustively.

Also remove Overworld's unimplemented resolveCollisions() method in the
header. It is unnecessary since the scene graph handles collisions itself.
Add another object to test.tmx

Test collision code for Mob by adding another Mob object to collide with.
It works successfully.