Notifications: use a single settings file.

There were 3 setting files, each an archived BMessage. Now there is only
one with all the data inside.
* Rework the SettingsPane class to save and load settings from a
message, rather than having each panel pick a file path on its own
* Move saving the app filters to the preference app, rather than the
server (so it's done at the same place as other settings)
* Rework loading prefs in the server so the settings message is read
from the file once and all settings are loaded from it.

This turns out to be more changes than I anticipated.
Fixes #9424.

headers/private/notification/Notifications.h

@@ -13,10 +13,7 @@
 const uint32 kNotificationMessage		= 'nssm';
 
 // Settings constants
-extern const char* kSettingsDirectory;
-extern const char* kFiltersSettings;
-extern const char* kGeneralSettings;
-extern const char* kDisplaySettings;
+extern const char* kSettingsFile;
 
 // General settings
 extern const char* kAutoStartName;