0.6.0

• Bump to master#73b3d7 branch
  • Also: Actualize saucer_webview_url function
  • Also: Actualize saucer_webview_handle_scheme function
• Added boson_get_backend function
• Added boson_get_backend_str function
• Added boson_error_code_canceled function

  Required to distinguish file selection errors from cancellation operations in dialog boxes