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
exportlayers: Change 'raw' extension to 'data'

This is to make the plug-in consistent with GIMP 2.8.14 (latest release as of writing this commit message). Also, minor cleanup.
pylibgimp: Add functions to remove all items
pylibgimp: Add custom duplicate function
exportlayers: Preserve image type and all of its metadata

The temporary image used to export layers does not contain metadata from the original image. This is required for e.g. indexed images. The solution is to duplicate the image, including all of its items (layers, channels and paths). These items have to be removed, since an empty image is desired. The performance penalty for in-memory item duplication shouldn't be high enough to warrant a different approach to copying image metadata.

This change has another side effect - image type (RGB, grayscale, indexed) is now preserved if possible, instead of forcing RGB mode all the time. Some file formats don't support some e.g. indexed types (e.g. PNG), so they automatically convert it to RGB or grayscale.
Readme, settings_plugin: Change 'raw' to 'data'
Changelog: Add entries for 2.2.1
pylibgimp: duplicate: Fix `remove_items` parameter
Changelog: Clarify entries for 2.2.1
Readme: Update installation and requirements
Readme: Update main description
Update page description
make_package: Process readme file

To avoid confusing the users, leading spaces are removed from terminal commands or suggested file contents.
make_package: Update .pot file generation

This should make sure that the .pot file is added to the release package.
Changelog: Update 2.2.1 release date
constants: Bump up version
make_package: Fix .pot file generation for real