tests: move libcomps test to a separate test file.
show plugin commands in separate block
api: add new public Command.usage & Command.summary API
cleanup: make Commands use the new api
test: use Command.usage & Command.summary API in unittest
cleanup: getSummary() and getUsage() can be dropped entirely now.

And a couple of other tweaks related to 0a6051b.
removed dummy_wrapper from yum/i18n.py
removed exception2msg from yum/i18n.py
removed str_eq from yum/i18n.py
removed utf8_valid from yum/i18n.py
moved _, P_ to dnf/i18n.py
make all strings unicode_literals implicitly
removed to_unicode from yum/i18n.py
removed utf8_width_fill from yum/i18n.py
removed utf8_width from yum/i18n.py
removed utf8_text_fill from yum/i18n.py
removed to_str from yum/i18n.py
removed to_utf8 from yum/i18n.py
fixed import errors after yum/i18n.py removal
added forgotten import
added --metadata-refresh option (RhBu:1064226)
added --metadata-refresh option to documentation