Remove the limitation that output formatters have to be exclusive

Introduce an aggregating output formatter that forwards its input to a
sub-formatter that feels responsible for it, or otherwise lets the base
class handle it.
Our output panes now use such an aggregating formatter.
In particular, this means that in the future, we won't have to stuff all
run control output formatting into the Qt output formatter anymore.

Change-Id: I5498f200a61db10ccff3ec8974c6825da7f7072d
Reviewed-by: hjk <hjk@qt.io>

src/plugins/serialterminal/serialoutputpane.cpp

@@ -315,7 +315,7 @@ void SerialOutputPane::createNewOutputWindow(SerialControl *rc)
             this, fontSettingsChanged);
     fontSettingsChanged();
     ow->setWindowTitle(tr("Serial Terminal Window"));
-    ow->setFormatter(formatter);
+    ow->setFormatters({formatter});
     // TODO: wordwrap, maxLineCount, zoom/wheelZoom (add to settings)
 
     auto controlTab = SerialControlTab(rc, ow);