Created the class EventTrigger and EventTriggerMode

Created the icon for EventTrigger

Created the schema files for SQL, XML and catalog queries for
EventTrigger
Added button to create Event Triggers on new object overlay.

Minor improvement on UI stylesheet
Finish creating the object EventTrigger (pending code comments)

Implemented the import of evet trigger from a database
Finish the implementation of EventTriggerWidget.

Quote the pgmodeler-cli command on ModelFixForm (issue #493)
Merge branch 'event-trigger-support' into develop
Schemas names are now correctly striked out on the rectangle when their
SQL code is disabled. The only exception is for public / pg_catalog,
even having the code disabled they will not have the names striked.

Enabled event trigger to accepts custom SQL commands (append / prepend)