LibGUI: Add a top-left-corner button to table views

If both the row and column headers are visible, we now also show a
button in the top left corner. This avoids the headers overlapping
each other when you scroll the contents.

In the future, this could be hooked up to a "select all" action.

Libraries/LibGUI/AbstractTableView.h

@@ -112,6 +112,7 @@ private:
 
     RefPtr<HeaderView> m_column_header;
     RefPtr<HeaderView> m_row_header;
+    RefPtr<Button> m_corner_button;
 
     HashMap<int, OwnPtr<TableCellPaintingDelegate>> m_column_painting_delegate;