Added input system.
Minor changes.

- Renamed get_owner to get_owning_[owner_type], still possibly too verbose. Considering
    get_[owner_type] and documentation of methods (to avoid inconsistancy with
    entity::get_component returning owned not owning components). Perhaps overcomplicating this.
- Removed code related to old input hack.
- Removed engine::remove_space until it's needed (not sure how it should be handled yet).
Minor changes.

- Removed now redundant vec[2-3]mk functions.
- Moved some magic numbers in debug_line_drawer into settings struct. Fixed circle drawing - was
    iterating one too many times.