FaceStore improvements and implementations.

- Better string comparison in FaceStore.GetFaceByName.

- Improved FaceStore.CreateFace so it won't try to create
  a face with the same name twice.

- Implemented FaceStore.LoadAllFaces, FaceStore.CreateTable,
  FaceStore.InsertFaceIntoTable, FaceStore.Remove and
  FaceStore.Commit.
Added more face tagging functionality.

- New FaceLocation and FaceLocationStore clases to implement faces
  with photos relationships.

- FacesPageWidget is a *singleton* now, so you can access it from
  anywhere —I need it at least to access the widget from the
  new FacePopup class... I don't like it a lot but I think it's
  far better than having faces-related methods in MainWindow as it
  happen with tags-related methods.

- FacesPageWidget is a VBox now, and I've started implementing a fake
  interface that shows you the already created test faces and a
  button —only if you are in ViewContext.Edit mode— that would create
  test faces if they don't exist yet. This is for testing database
  persistency and the ability to rename and delete those test faces
  with all the event handling stuff. But they don't appear in the photo
  in a graphical manner yet.

- FaceSelectionWidget is now a SaneTreeView instead of a regular
  GTK+ TreeView.
Updated German translation
Merge branch 'master' of git://git.gnome.org/f-spot into faces-tool
Updated Greek translation
Merge branch 'master' of git://git.gnome.org/f-spot into faces-tool