Enhancements to materialized view operations and insert/edit rows

• When dealing with materialized views, pass an extra parameter to Database controllers to let them differentiate between regular views and materialized views
• When inserting or editing table rows, if a given field has the null checkbox checked, add an event listener to uncheck the checkbox if the content field has changed and is non empty
• The method that draws the row update/insert form is now separate from the one that actually performs the update/insert. Previously it was the same method receiving a boolean flag.
• TODO: row update and insert logic are pretty similar. Such code should be moved to a trait that DisplayController and TableController would use to avoid useless repetition.
• TODO: row update shouldn't be part of DisplayController. It is intrinsecally a table operation and should be dealt with in TableController.