BUG: Slicer_EXTENSION_DESCRIPTION_DIR is a PATH, not a STRING

Slicer_EXTENSION_DESCRIPTION_DIR was declared as a STRING. cmake-gui
does not offer the possibility to open a file browser and select a
directory from the user interface if a variable is not defined as a
PATH or FILEPATH.

From: Francois Budin <francois.budin@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24988 3bd1e089-480b-0410-8dfb-8563597acbee