Add the 'const' attribute to let the compiler know that it doesn't
need to call it multiple times for the same debug level argument,
since the enabled log levels cannot change at runtime.

+ enable the new log writer on the executables
+ enable structured logging in the tests

This extends GLib's logging system, so it is compatible with
g_debug() and friends, but it uses a better logging format and
supports filtering debug domains with wildcards, like in gst.