add editable attribute to image component

do not run the open-assets command when editable==false

src/dom_components/model/ComponentImage.js

@@ -8,6 +8,7 @@ module.exports = Component.extend({
     src: '',
     void: 1,
     droppable: 0,
+    editable: 0,
     highlightable: 0,
     resizable: 1,
     traits: ['alt']

src/dom_components/view/ComponentImageView.js

@@ -45,7 +45,7 @@ module.exports = ComponentView.extend({
     var em = this.opts.config.em;
     var editor = em ? em.get('Editor') : '';
 
-    if(editor) {
+    if(editor && this.model.get('editable')) {
       editor.runCommand('open-assets', {
         target: this.model,
         onSelect() {