Spreadsheet: Add support for copying ranges of cells to other cells

Now the entire range is copied to the area around the target cell,
translating the current cursor to the target.

Applications/Spreadsheet/SpreadsheetWidget.h

@@ -52,6 +52,14 @@ public:
     Workbook& workbook() { return *m_workbook; }
     const Workbook& workbook() const { return *m_workbook; }
 
+    const GUI::ModelIndex* current_selection_cursor() const
+    {
+        if (!m_selected_view)
+            return nullptr;
+
+        return m_selected_view->cursor();
+    }
+
 private:
     explicit SpreadsheetWidget(NonnullRefPtrVector<Sheet>&& sheets = {}, bool should_add_sheet_if_empty = true);