Make the UIDataProvider more C++11. Range iteration.
Use nullptr instead of NULL
MetaDataWidget::add_data() short return if the propertyvalue is empty.
Only show error if error there is.

Clean up the code style a little bit.
Reset the view in the darkroom module when nothing is selected
Put up a placeholder.
Follow selection in the darkroom module. Fix a regression where selecting
again would reset the image.
Remove once again traces of Gegl from image.hpp.