keep default kit status in apply()

m_isDefaultKit is changed to wrong value during apply() process,we need
to keep the value so that the rest of the logic works correctly.

Task-number:QTCREATORBUG-8205
Change-Id: Id25e56273ed06957d0a26d893ec26b9bf73864fa
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/projectexplorer/kitmanagerconfigwidget.cpp

@@ -127,6 +127,8 @@ void KitManagerConfigWidget::apply()
 
     if (mustSetDefault)
         km->setDefaultKit(m_kit);
+
+    m_isDefaultKit = mustSetDefault;
     emit dirty();
 }