Integration of Pathfinding Code

Integrated Pathfinding code into the Game Loop.
- Calculates shortest path to all reachable locations on the map for the
currently selected Agent
- Calculates the total cost of moving the currently selected Agent all
reachable locations on the map
- Calculation is spread out over several seconds to maintain consistent
frame rate
- When an Agent is selected and a tile highlighted, the shortest path
found will be highlighted from the Agent's location to the highlighted
tile

Active Agents and (Inactive) Agents are now stored in separate
containers.
- Agents are now dynamically allocated
- Inactive Agents are no longer part of turn checks.
- Active Agents now store and handle data specific to them (player
owner, initiative, flags for began and ended turn)
- Inactive Agents can no longer be selected

Agents now have names
- Selected Agent's name is now displayed instead of identifying index

Added function to calculate the distance between two tiles.
Added functions to create Active and Inactive Agents, remove all
existing Agents.