Add "directly open editor" shortcut and icon

public/i18n/en_US.json

@@ -1,10 +1,12 @@
 {
   "insertDrawing": "Insert Drawing",
   "editDrawing": "Edit with js-draw",
+  "editShortcut": "Open editor directly",
   "errNoFileID": "File ID missing - couldn't open file.",
   "errSyncIDNotFound": "Couldn't find SyncID in document for drawing, make sure you're trying to edit a drawing that is included in at least a note.",
   "errCreateUnknown": "Unknown error while creating editor, please try again.",
   "errInvalidBackgroundColor": "Invalid background color! Please enter an HEX color, like #000000 (black) or #FFFFFF (white). The old background color will be used.",
+  "msgMustSelect": "Select a whiteboard in your document by left-clicking it, then use this icon/shortcut to open the editor directly.",
   "drawing": "Drawing",
   "settings": {
     "name": "js-draw Plugin Settings",