Added GeoClue2 privacy item

Rebased onto current master and fixed the privacy module build:
- privacy_item.hpp no longer includes privacy.hpp (it does not use the
  Privacy class); this broke the circular include that left PrivacyItem
  undeclared when privacy.hpp was reached first from privacy_item.cpp.
- privacy_item.cpp now includes gtkmm/label.h explicitly, since Gtk::Label
  was previously only pulled in transitively via privacy.hpp.

resources/style.css

@@ -325,3 +325,7 @@ label:focus {
 #privacy-item.audio-out {
     background-color: #0069d4;
 }
+
+#privacy-item.location {
+    background-color: #00afff;
+}