+ replace QDirModel with QFileSystemModel
+ QProcess::start(QString, OpenMode) is deprecated
+ QByteArray::append is deprecated
+ QPixmap* QLabel::pixmap() is deprecated
This patch includes the actual implementation of Link, which is
implemented as an extension named LinkBaseExtension in App namespace,
and a full view provider ViewProviderLink in Gui. The reason of not
using ViewProviderExtension is because it need full control when
display, not just extending existing functionalities.
Please see [here](https://git.io/fjPue) for more details of the
implementation.
This patch also includes a set of link manipulation commands, and a
task panel for overriding geometry element colors.
