Read embedded dcp profile from raw files

raw files, usually dng files, can provide a dcp profile by providing the
related tags.

This patch will make the "Use embedded" options in the color management
panel selectable if the source raw file embeds a dcp profile.
In this case the DCP panel will be enabled like when reading an external
dcp profile.

Additional changes:

* Rename "Use embedded, if possible" to just "Use embedded" since the
option is already disabled when no embedded profile is found.
* Update the "Use embedded" options tooltip as it's now not related to
  only non raw images.

rtgui/icmpanel.h

@@ -167,6 +167,7 @@ private:
     double dcpTemperatures[2];
     Glib::ustring lastRefFilename;
     Glib::ustring camName;
+    Glib::ustring filename;
     void updateDCP(int dcpIlluminant, Glib::ustring dcp_name);
     void updateRenderingIntent(const Glib::ustring &profile);
     void foldAllButMe (GdkEventButton* event, MyExpander *expander);