I worked on 'Relation Interface Improvements' feature during the first week of GSoC and was able to successfully implement it. Now the cumbersome task of setting relational constraints has been made somewhat easy and is less error-prone.
So, when the user selects the database, the entries in the table dropdown are filtered only to show tables in that database and column dropdown is filtered in a similar manner when a table is selected. Own database is selected by default as it's more common to setup relations inside the same database.
Checking relational integrity feature was already supporting cross database relations. I added table name to the displayed text to make it clearer. I also updated a couple of FAQs in the documentation related to these features.
For the next I have planned to implement 'Find and replace by column' feature which is quite long due. I have some preliminary work on it, and plan to finish implementing it during this week.