Move layouts definitions to backend and fix Firefox overlay scrollbars (#647)

* Fix two-axis scrollbars in scrollable regions on Firefox

* Move Document Mode dropdown to the backend; and related code cleanup

* Port the Layer Tree options bar layout to the backend

* Port the tool shelf to the backend

* Clean up initialization and wasm wrapper

* Fix crash

* Fix missing document bar

* Remove unused functions in api.rs

* Code review

* Tool initalisation

* Remove some frontend functions

* Initalise -> Init so en-US/GB doesn't have to matter :)

* Remove blend_mode and opacity from LayerPanelEntry

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

editor/src/layout/layout_message.rs

@@ -17,9 +17,12 @@ pub enum LayoutMessage {
 pub enum LayoutTarget {
 	DialogDetails,
 	DocumentBar,
-	PropertiesOptionsPanel,
-	PropertiesSectionsPanel,
+	DocumentMode,
+	LayerTreeOptions,
+	PropertiesOptions,
+	PropertiesSections,
 	ToolOptions,
+	ToolShelf,
 
 	// KEEP THIS ENUM LAST
 	// This is a marker that is used to define an array that is used to hold widgets