Show unit task queue and mark in side panel

Add `get_tasks` to the `Unit` trait and implement it for `MinerUnit`,
`Units` enum and `Option<Units>`.  Use the new method in `base_text` to
display a “Tasks Queue” line.

Refactor task assignment in `BoardState` to a chained call.

Update the skirmish view and `SidePanelWidget` to pass a flag indicating
whether the displayed unit is the currently marked one, and highlight
the
unit name accordingly.

src/app/states/skirmish_states/board.rs

@@ -220,9 +220,10 @@ impl BoardState {
 
         let (row, col) = self.marked_cells.selected_unit.get_coords();
         let task: Tasks = Tasks::Digging(DiggingTask::new(self.marked_cells.marked_cells.clone()));
-        let unit: &mut Option<Units> = self.get_mut_cell(row, col).get_mut_option_unit();
 
-        unit.set_task(task);
+        self.get_mut_cell(row, col)
+            .get_mut_option_unit()
+            .set_task(task);
 
         self.marked_cells.selected_unit = None;
         self.marked_cells.marked_cells.clear();