Move make_unique to its own header file

build/libaegisub/libaegisub.vcxproj

build/libaegisub/libaegisub.vcxproj.filters

libaegisub/common/file_mapping.cpp

libaegisub/common/io.cpp

libaegisub/common/json.cpp

libaegisub/common/option_visit.cpp

libaegisub/common/path.cpp

libaegisub/common/thesaurus.cpp

libaegisub/include/libaegisub/make_unique.h

libaegisub/include/libaegisub/util.h

libaegisub/unix/log.cpp

libaegisub/unix/util.cpp

libaegisub/windows/log_win.cpp

libaegisub/windows/path_win.cpp

src/ass_dialogue.cpp

src/ass_override.cpp

src/ass_parser.cpp

src/ass_style_storage.cpp

src/audio_display.cpp

src/audio_karaoke.cpp