Add Area::constrain_to and Window::constrain_to (#3396)

* Rename `drag_bounds` to `constrain_to`

and allow turning off constraining for `Window`s

* Constrain DatePicker popup to screen

* Fix typo

crates/egui_extras/src/datepicker/button.rs

@@ -117,6 +117,7 @@ impl<'a> Widget for DatePickerButton<'a> {
             } = Area::new(ui.make_persistent_id(self.id_source))
                 .order(Order::Foreground)
                 .fixed_pos(pos)
+                .constrain_to(ui.ctx().screen_rect())
                 .show(ui.ctx(), |ui| {
                     let frame = Frame::popup(ui.style());
                     frame