Merge experimental branch

[halildurmus]

• Adds winrtgen package (contains WinRT projection pieces extracted from tool\generator package)
• Some refactoring on the winrtgen package
• .gitignore, analysis_options.yaml, and dartdoc_options.yaml files are shared among all packages
• Adds the calendar example to the windows_globalization package
• Adds .pubignore and AUTHORS files
• Fixes the generated dartdoc category comments (e.g. /// @category Enum -> /// @category enum)
• Converts the helper functions in packages/windows_foundation/winrt_helpers.dart to camelCase (e.g. ActivateClass() -> activateClass())

... and much more!