Format string tweaks for translations, removing of mplayer code

Before posting the latest translations to crowdin, it's good to be consistent in format strings, so that translators won't get confused about what gets translated and what doesn't.  I migrated all `%s` format strings to `%(var_name)` format.

In the process, rather than fixing strings related to the embedded `mplayer` support, I simply ripped the support out.  We've never enabled this in production and, after 9 months of sitting in our code-base, I think it's fair to say we won't.

@dylanjbarth any concerns on the translation tweaks are appreciated; @jamalex any concerns on the code removal are appreciated.  Let's plan to merge this for tomorrow, so we can give translators 2 full weeks to translate before 0.12.0 is released!