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/units/miner.rs

@@ -57,6 +57,10 @@ impl Unit for MinerUnit {
         self.coords
     }
 
+    fn get_tasks(&self) -> String {
+        format!("{:?}", self.tasks)
+    }
+
     fn set_task(&mut self, task: Tasks) {
         self.tasks.push_back(task);
     }