Added Selection Grid system and refactored shop code to use it

include/selection_grid.h

@@ -0,0 +1,41 @@
+#ifndef SELECTION_GRID_H
+#define SELECTION_GRID_H
+#include <tonc.h>
+
+typedef POINT Selection;
+
+struct SelectionGridRow;
+struct SelectionGrid;
+typedef struct SelectionGridRow SelectionGridRow;
+typedef struct SelectionGrid SelectionGrid;
+
+// Called whenever there is a change in the selection cursor
+// row_idx is the index of the row whose function is invoked - can be used to identify whether it is the previous or new selection row.
+typedef void (*RowOnSelectionChangedFunc)(SelectionGrid* selection_grid, int row_idx, const Selection* prev_selection, const Selection* new_selection);
+typedef int (*RowGetSizeFunc)();
+// Called for any non-directional key hit
+// The key will not be passed, the function will have to check key_hit() etc. for the key it wants to check
+typedef void (*RowOnKeyHitFunc)(SelectionGrid* selection_grid, Selection* selection);
+
+struct SelectionGridRow
+{
+    int row_idx;
+    RowGetSizeFunc get_size; 
+    RowOnSelectionChangedFunc on_selection_changed; 
+    RowOnKeyHitFunc on_key_hit;
+};
+
+struct SelectionGrid
+{
+    SelectionGridRow* rows;
+    const int num_rows;
+    Selection selection;
+ };
+
+
+void selection_grid_process_input(SelectionGrid* selection_grid);
+
+void selection_grid_move_selection_horz(SelectionGrid* selection_grid, int direction_tribool);
+void selection_grid_move_selection_vert(SelectionGrid* selection_grid, int direction_tribool);
+
+#endif