Help: Avoid trying to open "Foo::bar" as a url in a browser.

Technically it is a valid URL with scheme "Foo" and path ":bar". With
this patch we only try to open URLs with very specific schemes like
"qthelp", "file", "http(s)", "about".

Task-number: QTCREATORBUG-7547
Change-Id: I6908ff554c12ca0f4cfc5bd0fb30dd659d780a23
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>

src/plugins/help/localhelpmanager.h

@@ -58,6 +58,7 @@ public:
     static BookmarkManager& bookmarkManager();
 
     static QVariant engineFontSettings();
+    static bool isValidUrl(const QString &link);
 
     Q_INVOKABLE QByteArray helpData(const QUrl &url);