common: conversion function from boost's ptime to time_t
mkvmerge: refactor muxing/writing app setting

Only set the muxing date once per run, not for each file.
cosmetics: alignment, commented-code removal
common/tags: in find by target type compare to target type value, not UID
cosmetics: unnecessary second if() removed
fixup! common/tags: in find by target type compare to target type value, not UID