Make code a bit more compliant with PEP 8

* removed trailing spaces in comments
* removed extra spaces around `dict` and `set` literals
* removed/added extra lines around functions, methods and classes
* re-indented several lines
pylibgimp: Add functions to remove all items
pylibgimp: Add custom duplicate function
pylibgimp: duplicate: Fix `remove_items` parameter
test_itemdata: Clarify _parse_layers
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
Rename library modules

Renamed several modules from `pylibgimpplugin` to clarify them and to avoid name conflicts (e.g. with variables).
test_pgsetting: remove redundant test case
pgpath: Module description clarification
gui_plugin: Add ability to press Enter to export layers from text field

While the focus is on the "File extension" text entry, pressing Enter will now export layers.
SettingPersistor: Make `status_message` a return value instead of a class member
SettingPresenter: Change properties to accessor methods

`value, `enabled` and `visible` are now accessor methods instead of properties for the sake of clarity - setting the `value` property was doing much more than merely setting a value. The other properties were converted to accessor methods for the sake of consistency.
Clean up source code documentation

* Removed method descriptions from the class docstring and put them into the respective method docstrings.
* Added an extra line between docstrings and the `pass` statements.
* Misc. documentation clarification.