Start of RefactoringService.
Fix features not being considered when determining height of container.
Fix usage of feature group type without checking whether is was valid.
Shuffle renaming code for features in preparation for implementation of
RefactoringService.

Prevent creating features using a Diagram as the Target Container
Fixes #47 and #51

Implements a RefactoringService that is responsible for renaming named
elements, updating related diagrams, and updating all references to the
named element.

Refactored code to use the RefactoringService.

Added handlers for a few other cases where references would not have
been updated appropriately.
Fixes #33.

Component Types and Implementation diagram root shapes and subcomponents
are no longer filled. This allows seeing the grid lines. As before,
grid-lines can be toggled via the View menu.

Implemented Memory shape using lines.