priority: set process priority to "background" for "--priority lowest" on Windows

This also lets Windows' scheduler select the "very low" I/O priority
class automatically. Implements #863.
priority: only select "idle" I/O class for "lowest" priority on Linux

This way behaviour matches on Windows and Linux.
man pages: update documentation for --priority
mmg: Add information about I/O priority to "priority" prefs tooltip
Translations: update list of translatables; update German
ChangeLog: update