This adds support for modifying the columns a constraint is applied on
in the Constraints tab of the Edit Table dialog. This is a major step
towards full constraint editing capabilities (adding new constraints,
modifying constraint type, and editing constraint type-dependent
parameters are the other missing pieces).
The reasoning behind the popup dialog is to not introduce another full
dialog on top of the Edit Table dialog. After opening the Edit Table
dialog, then switching to the Constraints tab, then opening another
modal dialog, I felt like losing track of what I am currently editing.
The popup dialog is certainly not great but feels a bit less intrusive.
Any suggestions regarding this are appreciated.
