ENH: Fix scene save with default parameters

Novice Slicer users often complain that scene saving fails. The problem
is that Slicer offers scene saving in a read-only directory by default
(into the current working directory, which is the read-only program
binary directory by default).

Fixes applied:

1. Changed the default scene root directory to be the user's documents
   folder (determined the same way as the default DICOM database directory).

2. Made default scene directory configurable in the application settings
   GUI (selection is stored in application settings under "DefaultScenePath" key).

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24771 3bd1e089-480b-0410-8dfb-8563597acbee