settings_plugin: Description clarification
export_layers: Description clarification
gui: Minor cleanup

* Renamed `GtkExportDialogDirectoryChooserWidgetPresenter` to `GtkExportDirectoryChooserPresenter` for the sake of brevity.
* Minor clarification.
objectfilter: Use get_subfilter instead of __getitem__

`get_subfilter()` is more consistent with other subfilter-related methods.
settings: Clean up Container

`Container` is now an abstract class. `__setitem__` and `__delitem__` were overridden in subclasses, only to be disallowed, so it made little sense to define them in the first place.
settings: Clarify documentation
Change "directory" to "folder" where appropriate

The term "folder" is now consistently used in the GUI. Among other things, a couple of settings were renamed to use "folder" instead of "directory".

It turns out there is a difference between "directory" and "folder" indeed:
* http://superuser.com/questions/169457/directory-vs-folders
* http://english.stackexchange.com/questions/113606/difference-between-folder-and-directory
* http://en.wikipedia.org/wiki/Directory_%28computing%29