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/mcusupportsdk.cpp

@@ -55,7 +55,7 @@ McuPackage *createQtForMCUsPackage()
                 McuPackage::tr("Qt for MCUs SDK"),
                 QDir::homePath(),
                 Utils::HostOsInfo::withExecutableSuffix("bin/qmltocpp"),
-                "QtForMCUsSdk");
+                Constants::SETTINGS_KEY_PACKAGE_QT_FOR_MCUS_SDK);
     result->setEnvironmentVariableName("Qul_DIR");
     return result;
 }