McuSupport: Register Qt for MCUs .qch files

At plugin initialization and along with the creation of a Qt for MCUs
Kit (which currently happens when the user presses "Apply"), these .qch
files get registered:

  <Qul_DIR>/docs/quickultralite.qch
  <Qul_DIR>/docs/quickultralitecmake.qch

In order not to duplicate the code for retrieving package paths (e.g.
the one for "QtForMCUsSdk") from the settings, some refactoring was
done.
As a result McuSupportOptions::qulDirFromSettings() has been introduced.
It will be also used in further changes.

Task-number: UL-1685
Change-Id: I82e638e129120cdadcf2f4812f467fce34b32ec9
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/mcusupport/mcusupportplugin.cpp

@@ -26,6 +26,7 @@
 #include "mcusupportplugin.h"
 #include "mcusupportconstants.h"
 #include "mcusupportdevice.h"
+#include "mcusupportoptions.h"
 #include "mcusupportoptionspage.h"
 #include "mcusupportrunconfiguration.h"
 
@@ -74,6 +75,7 @@ bool McuSupportPlugin::initialize(const QStringList& arguments, QString* errorSt
 
     dd = new McuSupportPluginPrivate;
 
+    McuSupportOptions::registerQchFiles();
     ProjectExplorer::JsonWizardFactory::addWizardPath(
                 Utils::FilePath::fromString(":/mcusupport/wizards/"));