Changed ImageView.ImageCoordsToWindow access from protected to public.

- Exposing ImageView.ImageCoordsToWindow enables external
tools —like the new Faces tool— to paint to the ImageView. Without this
method those external tools would need to duplicate code about
image orientation and image position in the gdk.Window, for
example.

Signed-off-by: Stephen Shaw <sshaw@decriptor.com>
New Sidebar.PageSwitched event.

- With this new event SidebarPages can know when the user is changing
between them.

Signed-off-by: Stephen Shaw <sshaw@decriptor.com>
Merge pull request #2 from Sanva/faces-tool

Faces tool