Add zindex to panel (#736)

* Add z-index options back to panel

* Add formatting for z-index panel buttons

* make z-index buttons all the same width

* make z-index button spacing even

* use svg icons & translations

* add ui legend

Co-authored-by: David Luzar <luzar.david@gmail.com>

src/locales/en.json

@@ -39,7 +39,8 @@
     "fileTitle": "File title",
     "colorPicker": "Color picker",
     "canvasBackground": "Canvas background",
-    "drawingCanvas": "Drawing Canvas"
+    "drawingCanvas": "Drawing Canvas",
+    "layers": "Layers"
   },
   "buttons": {
     "clearReset": "Clear the canvas & reset background color",