EOG currently uses GnomeAppBar for the statusbar, which is depracated.

It also has other problems:
- menu items tooltips are not shown in the statusbar as they should
- doesn't have a resize grip
- the progress bar is always visible even which there is nothing in progress

The attached patch adresses the problems above, by creating a very simple
EogStatusbar which inherits from GtkStatusbar.

Some notes:
- the resize grip is hidden when the window is maximized (this is what
eog_window_window_state_event is for)
- I moved the progress bar on the right, where in my opinion looks better (see
epiphany, firefox, etc)
- the progress bar is hidden by default and shown only during progess (this is
why I had to shuffle some of the gtk_widget_show/show_all in the window constructor)

At a later stage, if needed, it is easy to add furter messages like ("loading
file..." etc): just add a new message context and use gtk_statusbar_push()