Draft: ReplayGain support

I've added support for ReplayGain by using the rgvolume and rglimiter plugins. Those plugins are used in a gst::Bin that is set as audio-filter for the underlying playbin used by gst_player::Player.

I've also added a submenu to the menu to configure ReplayGain, allowing the user to choose between album mode, track mode, or no ReplayGain, as illustrated below.

Unresolved issues and limitations:

• Enabling or disabling the ReplayGain filter is only applied when the next song starts playing. However, switching between track and album mode is applied almost immediately.
• The submenu does not show which mode is currently enabled and still needs to be improved.
• Changing ReplayGain settings should not be possible when ReplayGain cannot be initialized (e.g. due to missing gstreamer plugins)

I would love to get some feedback. Let me know if there is anything you would like me to change.