Dependencies visualizer + UI enhancement

## Dependency Viewer

This pull request adds a viewer for the "callgraph" function dependency data contained in the dump-info dump file.  It displays this information in a 3-tiered table:
```
-------
-------
  +++++++
    =======
    =======
```
The selected element is in the middle, and the functions using that element are shown above.  The elements being used by the element are below.

## Paper-Elements

To accommodate the dependency viewer, the UI had to contain tabs to toggle between the different data models.  This either required either writing a complete tabs implementation, or using the tabs in Paper-Elements.  Because I was already using Polymer, this was an easy transition.
Done.