Support LUFS reference loudness values in replay gain plugin

Tools such as loudgain write ReplayGain 2.0 tags using a reference loudness of e.g. -18.00 LUFS. After being baffled for a while as to why the plugin didn't seem to be doing anything in Lollypop, debugging GStreamer yielded that this is being seen as a difference of 89 - -18 = 107 dB difference, likely because GStreamer always interprets replaygain_reference_loudness as dB.

It would be great if LUFS could also be supported, even if it is just by roughly converting it to dB in the background; LUFS -18.00 is approximately equal to the hardcoded 89 dB, at least according to the loudgain homepage above.

A workaround is to update the tags to not use LUFS.