#10 Allow unit orders from ingame UI

[Henning Thoele]

Acceptance criteria:
- Implement the following orders: Move, Attack, Harvest, Repair. The order shall be processed when performing a left-click
- Performing a right-click aborts the action
- Move and Attack shall result in the already implemented state change for the selected unit(s)
- Harvest and Repair may be performed, but shall not result in state changes (yet).
- When selecting an order, the cursor shall change to a crosshair when hovering over the map.
- When hovering over friendly entities, show green crosshair
- When hovering over empty or undiscovered terrain, or neutral entities, show yellow crosshair,
- Whene hover over hostile entities, show red crosshair