Entity world placing

[adam4813]

This involves the ability for a player to add, move, and remove entities to the world.

For this, engine systems will need to handle mouse click events and take appropriate action. Also editing widgets such as a rotation arc ball and translation cross-hair will need to be added as well.

• [x] Determine mouse click location within world
• [x] Create new entity at given location with selected properties
• [ ] Add collision check for valid placement
• [ ] Add shader to indicate valid placement

[adam4813]

Related #33 Material Picker and #38 Component packages.

[milesrout]

Definitely doesn't need to be in 0.1.

[milesrout]

Accidentally moved to 0.12, moved it back.

[adam4813]

Migrating this issue to being able to place an airlock in the world to serve as a starting point for creating ships. This provides a way to "connect" walls, floors, etc within the game world.