v0.5.2: Components update (#96)

• Material Progress: Cleanup animations on destroy to prevent memory leaks.
• Material Select:
  • Support deselect item in Material dropdown select with single selection
    model.
  • Unified template files (deleted material_select_dropdown_item.html).
  • Add support for Selectable SelectionOptions.
  • Create a ControlValueAccessor.
• Add backspace and delete keys to KeyboardHandlerMixin.
• Add selectedValue getter to RadioGroupSingleSelectionModel.
• Add null check to ObservableComposite's register method.
• Add totalEntitiesCountChange getter to table selection models.
• Add isStandardMouseEvent utility to test for clicks without modifier keys.
• Add string selection sanitizing options.
• Remove NoopStream in favor of Stream.empty() as provided by the SDK.
• Migrate use of LazyEventController to StreamController.
• Fix bug in lazy group creation that would cause multiple groups to be
  created.
• Strong mode fixes.