Add a preference for controlling the last calendar the user entered.

[jwir3]

This commit actually does a bit more than the above. It also restructures all user-related preferences into its own object on the PreferenceStore, named (aptly) "user". Finally, it organizes the preference store keys a little better and changes the names of the objects used to access these keys, for clarity.

Issue References:

• Fixes #18.

Development/Code Review Checklist

• [x] Documentation updated
• [x] Tests complete successfully
• [x] Formatting conforms to code style guidelines