ISIS-553: updates to example todoapp to demonstrate ViewModel functionality

* new ToDoItemsByCategoryViewModel
* new ToDoItemAnalysis and ToDoItemAnalysisContributions

in addition:
* reorganized the menu items a little ("Prototyping" menu instead of "Fixtures")
* renamed QuickStartApp -> ToDoApp etc
* moved app.* to webapp.* in the webapp module
  (because new view model stuff is now in an 'app' package in the dom module)

also:
* fixed styling issue with multiline text strings
* added AbstractContainedObject#removeIfNotAlready(...)