[UI] Clean NativeAppWindowViews

Remove unnecessary forward declarations, includes and ifdefs. Now that
Windows is using Aura, all the platforms using Views system (Win, Linux
and Tizen) are using Aura, so no more ifdef guards for USE_AURA.
Merge pull request #1069 from cmarcelo/clean-native-app-window-views

[UI] Clean NativeAppWindowViews