Define scheme handler for help, main and info URLs

This will allow khelpcenter to use the standard XDG scheme handler
mechanism instead of various custom URL handling.

org.kde.Help.desktop

@@ -194,3 +194,4 @@ Comment[zh_TW]=瀏覽並搜尋文件
 Categories=Qt;KDE;Core;
 OnlyShowIn=KDE;
 X-DBUS-ServiceName=org.kde.Help
+MimeType=x-scheme-handler/help;x-scheme-handler/man;x-scheme-handler/info;