Refactor cell marking, add undo, update keybindings

CellWidget::set_selected and set_marked now return &mut Self for method
chaining. Added BoardState::undo_marked_cell to remove the last marked
cell and restore focus. Backspace triggers undo, Delete clears marking.

src/app/views/skirmish.rs

@@ -27,6 +27,7 @@ const ACTIONS: &[Action] = &[
     Action::VolumeDown,
     Action::Space,
     Action::Backspace,
+    Action::Delete,
     Action::Mute,
     Action::Quit,
     Action::Quit2,