Revision 2544

Rework the status bar updates a bit to make sure the message about
the projections is produced at the right times.

* Thuban/UI/mainwindow.py (MainWindow.update_status_bar_messages):
New class variable with messages that may require a status bar
update.
(MainWindow.view_position_changed)
(MainWindow.update_status_bar): Rename from view_position_changed
to update_status_bar.  It's meaning has changed now that it may
also generate messages about problems with projection settings.
(MainWindow.__init__): Use the new update_status_bar_messages
class variable to subscribe update_status_bar
(MainWindow.set_position_text): Update doc-string.  This method
has to be renamed at some point.  See doc-string and comments.
(MainWindow.OnClose): Unsubscribe update_status_bar from all
messages in update_status_bar_messages

* Thuban/UI/viewport.py (ViewPort.forwarded_map_messages): New
class attribute.  map messages to be forwarded by the viewport.
(ViewPort._subscribe_map, ViewPort._unsubscribe_map): (un)subscribe
the messages in forwarded_map_messages