Create an assets panel

[vincentfretin]

Create an assets panel listing the currently registered assets from a-assets, and in the future the templates #28

Note that the current ModalTextures.js already list all a-assets img so we should reuse that code here, but also supports showing other assets like video, audio and a-asset-item (glb)

[vincentfretin]

New assetcreate, assetupdate, assetremove commands were done in #78 with a specific modal listing the materials assets.