IMediaLibrary: Provide a success state to onEntryPointAdded

For consistency with other callbacks, and because we might try to add a
new entry point but fail to do so.
The callback was also emited a tiny bit too early
4 jobs for add_success_to_entry_point_added in 11 minutes and 6 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #282530
amd64 debian
asan-ubsan

00:11:06

passed #282527
amd64 debian
linux

00:09:06

passed #282528
win32
win32

00:10:00

passed #282529
win64
win64

00:11:01