[Suggestion] Schema/Table Group Support

I just moved my project from DBDiagram.io to drawDB (since it is easier for newcomers, and **it is awesome!**)

After I use it for a while, I would like to provide feedback based on my use case. One feedback is already mentioned in #12. Another one is related to schema/table group support.

For me, my biggest feature that could improve my workflow drastically is to include schema support for each table. Before you may ask, yes, subject area is a workaround for that.

But I think it would be easier to select schema, then create table inside schema, and group them inside the same schema area.

I understand that this feature is quite complex, honestly. However, it should improve my existing workflow that involved with several schema and a ton of tables.