common/button library

1. files_preview package
2. documentation
3. common/button.dart

files_preview package

1. Libraries
2. common/app_bar
3. common/button
4. common/common_wrapper
5. files_preview
6. files_preview_method_channel
7. files_preview_platform_interface
8. image
9. photo_view
10. photo_view_gallery
11. video_full_screen