Create a new snackbar element to wrap the use of the mwc-snackbar

nuxeo-app element is currently handling all the notifications and toasts in Web UI and the code base is getting bigger and hard to maintain. 

Our goal is to create a new snackbar/toast element to wrap the use of the mwc-snackbar. This element has multiple purposes:

1. manage multiple snackbar instances
2. wrap the use of the mwc-snackbar
3. hide the necessary hacks to display multiple mwc-snackbar in Web UI