FlipView Patch #1

Various changes/bugfixes:
- Fixes #485
- SelectedIndex changes:
    - When the ItemsSource has changed, reset the SelectedIndex to 0.
    - Made sure SelectedIndex was 0 when the FlipView's template is applied.
- FlipView accepts the Left and Right arrow keys for going Backward and Forward respectively.
- FlipView's inner controls can no* longer receive tab focus.
- Added functions for hiding and showing the control buttons (i.e. Backward Arrow Button and Forward Arrow Button).
- Fixed a flicker bug of the banner's text when the banner's enabled status is toggled.
- Fixed a bug caused by the TabControl's default behavior. Control's Loaded event can be fired twice because of it.