preferences dialog.

Also moved function prototypes and macro definitions to top of
file.

* src/ntl-window.c: (impl_Nautilus_ViewWindow__create),
(nautilus_window_remove_meta_view):
Removed the *_real cruft.  Cleaned this a tiny bit.

(window_find_sidebar_panel_by_identifier):
New function for finding panels given an identifiers.

(window_update_sidebar_panels_from_preferences):
New function to update the current list of sidebar panels from
preferences.
Disabled panels are removed if they are already in the list.
Enabled panels are added if they are not already in the list.

NOTE: It would be 'nice' if in the future all updates of the
sidebar panel occur in just one place - the above function for
example.  This is in contrast to the current mess where both
ntl-window.c and ntl-window-msgs.c abuse the meta_views list at
random places.  Ill look into this later.

(sidebar_panels_changed_callback):
Now actually responds to changes.

* src/ntl-window.h:
Style dumbness.