COMP: Move Editor icons from "Base/Logic" into "EditorLib/Resources/Icons"

Also introduced the python constant EditorLib.ICON_DIR allowing
to easily resolve icon filepaths.

The variable is set using the __file__ attribute of EditorLib module.

Since the file/directory organization of either (1) source code, (2) file copied
into the build dir or (3) installed files is the same, the ICON_DIR
variable will allow to resolve the icon file paths successfully.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21765 3bd1e089-480b-0410-8dfb-8563597acbee