Change the definition of clicked_by (#4192)

This is a refactor on the way to add support for opening context menus
on touch screens via press-and-hold.

This PR changes what `InputState::button_clicked` does (it was ver badly
named before), and also changes `Response::clicked_by` to no longer be
true if clicking with keyboard (i.e. a widget has keyboard focus and the
user presses Space or Enter).

crates/egui_demo_lib/src/demo/pan_zoom.rs

@@ -11,7 +11,7 @@ impl Eq for PanZoom {}
 
 impl super::Demo for PanZoom {
     fn name(&self) -> &'static str {
-        "🗖 Pan Zoom"
+        "🔍 Pan Zoom"
     }
 
     fn show(&mut self, ctx: &egui::Context, open: &mut bool) {