Select all action doesn't select all diagram elements.

[philip-alldredge]

The select all action provided by GEF only selects top level diagram elements. This is problematic because it makes it difficult to select all elements to adjust contents.

We need to override the Select All implementation with an action that will select all elements similar to how Select All Connections works. This will bring the behavior in line with other common tools.

[philip-alldredge]

@brlarson this will enable the intended workflow of Select All/Ctrl-A and then Show/Hide Contents to adjust the contents of all elements.