Config class improvements (#826)

* Subclass ConfigParser
* Remove redundant variables and update tests
* Update file description
* Move NWConfigParser class to common.py
* Improve coverage of config class
* Use NWConfigParser also in themes.py
* Update config class file header
* Fix comment
* Update the usage of ConfigParser to recommended practice and extend tests
* Improve logging for info level a bit

nw/core/index.py

@@ -257,10 +257,10 @@ class NWIndex():
 
         # If the file is archived or trashed, we don't index the file itself
         if self.theProject.projTree.isTrashRoot(theItem.itemParent):
-            logger.info("Not indexing trash item '%s'", tHandle)
+            logger.debug("Not indexing trash item '%s'", tHandle)
             return False
         if theRoot.itemClass == nwItemClass.ARCHIVE:
-            logger.info("Not indexing archived item '%s'", tHandle)
+            logger.debug("Not indexing archived item '%s'", tHandle)
             return False
 
         itemClass  = theItem.itemClass