dbhub: Use certificate manager to get client cert for authentication

When trying to fetch a database don't load a hardcoded client
certificate but grab one from the certificate manager in the preferences
dialog.

Also add support for password protected private keys.

Note: Since this is all testing code on the front-end side, we just use
the first certificate in case multiple certificates are configured.

src/RemoteDatabase.h

@@ -23,7 +23,7 @@ public:
     const QList<QSslCertificate>& caCertificates() const;
     const QMap<QString, QSslCertificate>& clientCertificates() const { return m_clientCertFiles; }
 
-    void fetchDatabase(const QString& url);
+    void fetchDatabase(const QString& url, const QString& clientCert);
 
 signals:
     void openFile(QString path);