Convert the OSX-specific WebView extension code to ObjectiveC++

[rdw-software]

Maintaining the low-level C code that uses the ObjectiveC runtime directly is problematic. There isn't much of it, yet, but I can already feel the pain. If more WebView-based features need to be implemented, or various (especially OSX-specific) issues need to be fixed, there's no way around having a simpler way of writing OSX code. Getting compile-time checks would also be a huge boon.

They presumably chose this approach for an increase in portability, but in the context of this project that's completely irrelevant.

[rdw-software]

This is mostly blocked by the WEBVIEW_HEADER issue mentioned in #201 . It's easy to make it work by hacking around it, but a clean multiplatform solution just isn't in sight with the limitations imposed by the webview library's design. Not great, but alas.

[rdw-software]

Should be able to resolve this after the migration to a customized fork of webview (#201) is completed.