This allows the user to know what's contained in a playlist without
listing the media, and will allow the application to filter playlist by
their content type
Refs #235

Fix #321

This matches what's done for the removal

It will update more than just the media count in the next commit

Fix #268

This being a valuable performance boost, especially with long strings,
which is usually the case for the SQL request cache

This saves a temporary std::string construction/destruction each time
the hook is invoked, which is at least once per modification request,
and more when triggers are involved

This will allow heterogeneous lookups in std::unordered_map in order to
remove the need of creating a temporary string each time we perform a
lookup with a const char*

This mostly fixes cppcheck warnings about value not being read

Fix #370

Which can be used to trigger multiple runs of the same discovery, in
order to collect more profiling data

We always provide all the meta after preparse, and even if we weren't
storing all of those, we just return a default initialized string when
the meta wasn't stored.
This can be done with an std::array which costs nothing at runtime

Expose a single version that takes the string as value and expect the
value to be moved inside the parameter
This doesn't change the performances much but we now have a single
overload to maintain which clarifies the code a bit