`Ui::response` only works if `UiBuilder::id_salt` is uniquely set

[emilk]

• Related: Interactive Ui:s: add UiBuilder::sense and Ui::response #5054

The widget id used for the interact/response of Ui is currently Ui::id, which has a problem: is not necessarily unique.

We should instead use an Id that is globally unique, i.e. based on where in the hierarchy the Ui is, i.e. based on next_auto_id_salt. I believe this means we need to store this in Ui so we can use the same Id in remember_min_rect.

To see the problem with the current code, just apply this diff:

--- a/crates/egui_demo_lib/src/demo/interactive_container.rs
+++ b/crates/egui_demo_lib/src/demo/interactive_container.rs
@@ -21,6 +21,7 @@ impl crate::Demo for InteractiveContainerDemo {
             .show(ctx, |ui| {
                 use crate::View as _;
                 self.ui(ui);
+                self.ui(ui);
             });
     }
 }

This also means the drag-and-drop demo uses the same Id for all drag-sources, which exsasserbates

• Ui::with_layer_id panics when given LayerId is different from the Ui's current one. #5178