Feature/bugfix refactor

fixes:

• uncaught exceptions (according to the number of manifests) caused by this.$router.push()
• itemurl and imageurl are assigned each time when getManifests() is called. so itemurl becomes the last item on initial load
• delete obsolete structure sequences. data is already hold in manifests-object
• fix getLabel() for single Manifest
• create mixin for navigation as both, navbar and navbar-extended share the same props and methods
• combine files cssclasses.json and icons.json into one file: assets.json
• apply styles
• imageurl is loaded and updated every time, when a treenode is clicked. The node is supposed to just open
• OSD: commenting lines @18146,@18147

// context.mozImageSmoothingEnabled = this._imageSmoothingEnabled;
// context.webkitImageSmoothingEnabled = this._imageSmoothingEnabled;

and lines @18412,@18413

// $.console.assert(bounds.width > 0, "[Viewport._setContentBounds] bounds.width must be greater than 0");
// $.console.assert(bounds.height > 0, "[Viewport._setContentBounds] bounds.height must be greater than 0");

to eliminate console warnings