bug#26988: Add desktop-file-utils to all gtk applications

[Danny Milosavljevic]

Hi,

these are two patches I propose we add in order to add desktop-file-utils to 
all gtk applications.  This means that the profile hook would pick up the 
desktop files (as soon as possible).

One patch moves desktop-file-utils from (gnu packages gnome) to (gnu packages 
freedesktop).  I suggest we apply this patch to all branches - since they don't 
change anything in the build.

The other patch adds desktop-file-utils as propagated-input to gtk+-2 (and by 
inheritance to gtk+ 3, too).  That would imply entail all gtk applications.  
Therefore, it should only go to core-updates.