[GUI] Make UrlLabel in About styleable

The UrlLabel in the About FreeCAD box was not styleable because its
rich-text contents overrode any styling applied to it. This commit
converts it to a plain text label that can then be styled in a
stylesheet using the Gui--UrlLabel selector. If no stylesheet is
applied, the UrlLabel styles itself using the old-school blue text with
an underline, but any applied stylesheet completely overrides this
default.

This does not affect the "clickability" of a UrlLabel widget, which is
handled independently of the label's content (including its textual
hyperlink, which was and still is ignored).

src/Gui/Widgets.cpp

@@ -825,6 +825,10 @@ UrlLabel::UrlLabel(QWidget * parent, Qt::WindowFlags f)
 {
     _url = QString::fromLatin1("http://localhost");
     setToolTip(this->_url);
+
+    if (qApp->styleSheet().isEmpty()) {
+        setStyleSheet(QString::fromLatin1("Gui--UrlLabel {color: #0000FF;text-decoration: underline;}"));
+    }
 }
 
 UrlLabel::~UrlLabel()