Adds a view model for interval activities. A view model will extends a
data model with view-specific properties and operations.
Fixes a typo in the class documentation.
Adds an activity to interact with an interval activity. Adds a view and
some strings to support this view. Also, this new activity is
temporarily the main view for this application while it is being
implemented.
A list of app colors has been added. I am starting to create some styles
for the app views.