BUG: Fixed default DICOM database directory

When Slicer is installed on a computer the first time it set the DICOM directory to a read-only directory (silently) and all DICOM imports were failed (silently).

Root cause: ctkDICOMBrowser is created before the DICOM module checks the DICOM directory (and initializes it if empty). This is an issue, because ctkDICOMBrowser constructor sets the DICOM directory to be in the current working directory if it is not set in the settings. The current directory is typically C:\Program Files..., which is read-only.

Solution: check the DICOM directory and initialize it properly before creating the ctkDICOMBrowser object.

Also added a check to warn the user if a non-empty directory is selected (if that is not a previously created DICOM database directory). This might prevent users from trying to import DICOM files by setting the DICOM database directory to the location where those files are stored (it would not import the files anyway).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24624 3bd1e089-480b-0410-8dfb-8563597acbee