Fix infinite loop when adding a column to an existing primary key - #4217

Table::addKeyToConstraint() inserts the extended constraint into the
multimap while iterating over it. The column list [a,b] sorts after [a], so
the loop finds the same constraint again and extends it forever, which
froze the application and ended in a bad_alloc when ticking a second
primary key checkbox in the Edit Table dialog. removeKeyFromConstraint()
has the same pattern. A constraint object is only stored once, so stop
after updating it.

A new unit test covers adding and removing keys; it hangs without this
fix.

Fixes sqlitebrowser#4150