Add progress tracking to tasks and turn advancement

- Extend the `Task` trait with methods for progress handling and
  completion.
- Update `DiggingTask` to store `progress` and `tick_increase`, and
  implement the new trait methods.
- Implement the new methods in the `Tasks` enum, adding a mutable
  accessor.
- Introduce `BoardState::get_active_tasks` and
  `BoardState::advance_turn` to iterate over active tasks and update
  their progress each turn.
- Un-comment the call to `board.advance_turn()` in
  `SkirmishState::tick_update`.

src/app/states/skirmish.rs

@@ -11,7 +11,7 @@ pub struct SkirmishState {
 
 impl SkirmishState {
     pub fn tick_update(&mut self) {
-        // self.board.advance_turn();
+        self.board.advance_turn();
 
         // if self.board.is_victory() {}