Make the Pen tool only append new paths when Shift is held (#2102)

* Append to a path with shift

* Fixup transforms

* Revert unnecessary transform change

* Fix delete node button transaction

* Prevent artboard from being selected after making a new document

---------

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

editor/src/messages/portfolio/document/navigation/navigation_message_handler.rs

@@ -321,6 +321,7 @@ impl MessageHandler<NavigationMessage, NavigationMessageData<'_>> for Navigation
 				let diagonal = pos2 - pos1;
 
 				if diagonal.length() < f64::EPSILON * 1000. || ipp.viewport_bounds.size() == DVec2::ZERO {
+					warn!("Cannot center since the viewport size is 0");
 					return;
 				}