non-group-files will be loaded much faster now

since now, e.g. with tiff files, each and every file tried to examine all
files in the same folder to check whether there exists a metadata file
in the same directory. this check was performed on setSource,
even if m_groupfiles was disabled.

Signed-off-by: dietzc <christian.dietz@uni-konstanz.de>
Merge branch 'master' into trackhilite