Add the concept of "propel" to entities

Currently entities only have a concept of "velocity". This is set by the client and determines how the entity will move.
However, as soon as there's a collision this breaks down because the collision will alter the velocity. The new velocity will be a deflection (like when a ball is thrown against a surface).
For self propelled entities we however don't want a deflection, we instead want a combination of the self applied force and the force from the collision object, allowing for the entity to instead "slide" against anything it collides with.

To do this we need to add a new "propel" property separate from the "velocity" attribute.