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/mcusupportoptionspage.cpp

@@ -223,14 +223,15 @@ void McuSupportOptionsWidget::showEvent(QShowEvent *event)
 
 void McuSupportOptionsWidget::apply()
 {
+    m_options.qtForMCUsSdkPackage->writeToSettings();
     for (auto package : m_options.packages)
         package->writeToSettings();
 
-    QTC_ASSERT(m_options.qtForMCUsSdkPackage, return);
-
     if (!isVisible())
         return;
 
+    McuSupportOptions::registerQchFiles();
+
     const McuTarget *mcuTarget = currentMcuTarget();
     if (!mcuTarget)
         return;