Add Support For Composite Keys

[candrewlee14]

Maybe I'm just not seeing this feature, but I'm having trouble finding an option to create a multicolumn composite key.
It would be convenient if you were to be able to highlight multiple rows, right click, and create a composite key.

The graphic for this could have a small fork with indicators touching the columns in the key, and a key icon at the end of the stem of the fork.

[Chahk]

Same for Unique constraints please! Right now enabling "Unique" for more than one column on a given table will create multiple constraints.