Make rate applying more consistent and give more information

This implements the ideas discussed in http://www.darktable.org/redmine/projects/darktable/wiki/SelectionProposal
I am not directly committing it because 1) I don't know if I am breaking the string freeze with the added (copied) messages and 2) the conditionals set on the imgid to apply ratings to selected images or image under mouse should also be done in colorlabels and in any lib on the right panel affecting images.