visitor pattern in database

Added stubs of classes to implement the visitor pattern for database
access
Initial support for tabs
Remove tabs and canvases when closing the project.
Add an Element Visitor base class and implement an XML writter for. Added unit tests for the EVisitor and Element packages. More tests are needed.
XMLReader

EVisitorXMLReader.java basics written.
EVisitorTest class augmented to use reader
Create elements with the gui and draw them
Create elements and draw
drawing rectangles and lines
clean up.
Merge branch 'Drawing' into new_diagram

Conflicts:
	src/GUI.java
update drawing classes to look like a class, and relationships to look like their relationship.
Merge branch 'Drawing' into new_diagram

Conflicts:
	src/GUI.java
Merge remote-tracking branch 'origin/ched_database' into new_diagram

Conflicts:
	src/Element/Diagram.java
	src/Element/Element.java
	src/Element/ElementClass.java
	src/Element/ElementRelationship.java
	src/Element/ElementRelationshipAggregation.java
	src/Element/ElementRelationshipAssociation.java
	src/Element/ElementRelationshipDependency.java
	src/Element/ElementRelationshipGeneralization.java
Add back tabs
Merge pull request #7 from Zedzap/new_diagram

Merge the Drawing, ched_database, and new_diagram branches back into master.