Change time-related Movie glib API's to return time as out

Also, added Since: strings and completed missing API doc comments.
6 jobs for issue-777 in 4 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #401188
build

00:03:30

passed #401193
build_android

00:01:12

passed #401189
build_clang_libcpp

00:03:30

passed #401192
build_clazy

00:04:43

passed #401191
build_mingw64_fedora

00:04:36

passed #401190
build_ubuntu_16_04

00:02:26