Fixed bug when deleting tables and fk rels. together (issue #401)

Created the Tag class that will be used to classify views/tables.
Finished the implementation of class Tag.
Added an graphical element to tables/views to represent tags.

Added entries at AppearanceConfigWidget config combo to handle tag
colors and font style.

Minor change on table and view classes to add tags to it's xml
definition.
Implemented the form to edit/create tags.

Add tag selectors to view and table forms.

Minor change on OperationList class to handle tags.
Finished the implementation of tag support (issue #394)

Removed commented and duplicated code from ModelObjectsWidget.
Minor fix on tag editing form to update references to the handled tag.
Merge branch 'feature394-table-tags' into develop