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>

.vscode/settings.json

@@ -3,7 +3,7 @@
 	"[rust]": {
 		"editor.formatOnSave": true,
 		"editor.formatOnPaste": true,
-		"editor.defaultFormatter": "rust-lang.rust-analyzer",
+		"editor.defaultFormatter": "matklad.rust-analyzer",
 	},
 	// Web: save on format
 	"[typescript][javascript][vue]": {