Ability rework.

See merge request !25

- Handles Abilities and controlling them.

- Uses AttackStats passed from the attack.
- Takes target from attack instead of mouse pos.
- Implemented range.

PrefabService & abilities

See merge request !24

- It loads all prefabs into a dictionary at boot.
- Handles instantiation and destruction of prefabs.

- Added method for healing characters.
- Added calls to trigger some item events.

Created ItemService.

See merge request !23

- Now creates a new instance of the item instead of returning the same one.
- Grabs all implementations of the item class dynamically at start.

Implemented initial version of conditional items.

See merge request !22

- Added code to trigger the events.
- Added Actor prop to AttackStats so we can see where the damage came from.

- Also moved some classes around to avoid future clutter.

- Added code to Inventory to inform owner of conditional items.
- Added code to Character to add/remove listeners.

Updating master to have the new stuff from main.

See merge request !21

New items system.

See merge request !20

- Items class hierarchy.
- Inventory class.
- CharacterStats class.
- Applies modifiers to character's stats.