Quicklook thumbnailing extension for webxdc files

To see webxdc app icon instead of zip file icon.

MacOS

• seems still to be kinda slow (sometimes thumbnails are not loading fast enough in finder, sometimes it blocks other thumbnails too)
  • is it the unzipping, image loading, bad code, debug build or sth else?
• we can compile it and then put the .appex into a special dir in dc desktop, but maybe we need some resigning magic for it to work in the end

iOS

To build for iOS you need to change in thumbnailing extension:

• base sdk -> to iOS
• buildphases -> link with libraries -> remove quartz framework

Note that to test changes properly on iOS, you need to install/refresh the parent app, injecting/running just the extension does not work for some reason.