Sets whether to show a drag handle in the bottom sheet.
The handle will be overlaid over [property@BottomSheet:sheet].
When the handle is shown, [class@HeaderBar] will hide its default title, and [class@ToolbarView] will reserve space if there are no top bars.
Showing drag handle also allows to swipe the bottom sheet down (and to swipe the bottom bar up) with a pointer, instead of just touchscreen.
| this |
a bottom sheet |
| show_drag_handle |
whether to show the drag handle |