Add feature to collapse group

[ask9]

In order to cope with the ever-increasing complexity of schemas in databases, I suggest adding the feature to collapse groups of tables.
In the right upper corner in rectangle of each group the small button with arrows appeared.
The content of the group is collapsed/expanded on mouse click on this button.

It can be useful if the user does not want to see too much detail about specific tables in the groups, if they are not important now, but wants to conceptually understand the relationships between groups, and does not want to exclude them from the scheme altogether.

Here is a simple presentation in gif-file: