Add auto-panning when pointer goes beyond viewport edge with Select tool (#1625)

* Add code to shift viewport if mouse is beyond edge

* Allow shifting viewport if mouse is stationary too

* Group all modifier keys into SelectToolPointerKeys

* Cleanup message ordering to remove shifting during resize

* Slow down shifting by half

* Clamp speed; code review cleanup

---------

Co-authored-by: 0hypercube <0hypercube@gmail.com>
Co-authored-by: Keavon Chambers <keavon@keavon.com>

editor/src/messages/broadcast/broadcast_event.rs

@@ -5,6 +5,7 @@ use serde::{Deserialize, Serialize};
 #[derive(PartialEq, Eq, Clone, Debug, Serialize, Deserialize, Hash)]
 #[impl_message(Message, BroadcastMessage, TriggerEvent)]
 pub enum BroadcastEvent {
+	AnimationFrame,
 	CanvasTransformed,
 	ToolAbort,
 	SelectionChanged,