Group objects in the DB structure tab

Group the database objects by their type when inserting them into the
tree widget in the structure tab.
Rewrite the find dialog using Qt Designer

Create a Qt Designer form file for the find dialog.

Simplify the find dialog code a little bit.

Add a new comparison operator '<>' in the find dialog.
Rewrite EditDialog using Qt Designer

Create a Qt Designer form file for the edit dialog.

Clean up the edit dialog code removing some not working and not used
functionality e.g. for blob editing.