Rename marking methods and use get_coords

Introduce push_marked_cell, pop_marked_cell and end_marking_cells to
replace the
previous set_marked_cell, undo_marked_cell and clear_marked_cells.
Collapse
get_row/get_col into get_coords and update all callers accordingly.

src/app/views/skirmish.rs

@@ -130,8 +130,7 @@ pub fn skirmish_view(app: &App, area: Rect, buf: &mut Buffer) {
         BoardWidget::new(&board).render(cells_area, buf);
 
         if states.skirmish.side_panel {
-            let row: usize = board.get_focused_cell().get_row();
-            let col: usize = board.get_focused_cell().get_col();
+            let (row, col) = board.get_focused_cell().get_coords();
             let mut is_unit_marked: bool = false;
 
             if let Some((first_row, first_col)) = board.marked_cells.marked_cells.get(0) {