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:
- Changed the default scene root directory to be the user's documents folder (determined the same way as the default DICOM database directory).
- Made default scene directory configurable in the application settings GUI (selection is stored in application settings under "DefaultScenePath" key).