Connect button should be disabled by default

Since ConnectDialog::accept calls are refused if no item is selected,
and the server last connected to is selected in ConnectDialog::timeTick
(after initialisation), it doesn't make sense for the Connect button
to be enabled before a server is selected.

src/mumble/ConnectDialog.cpp

@@ -821,6 +821,7 @@ ConnectDialog::ConnectDialog(QWidget *p, bool autoconnect) : QDialog(p), bAutoCo
 		qlPublicServers.clear();
 	}
 
+	qdbbButtonBox->button(QDialogButtonBox::Ok)->setEnabled(false);
 	qdbbButtonBox->button(QDialogButtonBox::Ok)->setText(tr("&Connect"));
 
 	QPushButton *qpbAdd = new QPushButton(tr("&Add New..."), this);