Synchronise configuration

[clanghout]

The configuration is currently stored in localstorage, which means that if you use the dashboard on another device, you start with a blank grid.
Session storage is an option to use. This can be used to store data that will be synchronised using a google account.

The problem is that the configuration I am currently using produces json that is too big (in file size) to immediately save to session storage.

The json can possibly be trimmed, but will not fix the entire problem in the case people want to load in complete configurations.

[clanghout]

My current thought is making a list of actions that can be pushed to the session storage from the localstorage. This will also mean that a history will be saved that can be used to undo changes.

[clanghout]

Also a merge might be necessary if different configurations are pushed to the session storage.

Basing the storing on events will be a fix, if the newly added widgets do not fit in the current config, gridstack should automatically reposition them.

[clanghout]

See #12