Consistent processor labels "Meter"

Track name is implicit, so instead of "meter-<name>", showing a
translatable label "Meter" is sufficient and consistent with "Fader".

Under the hood, for introspection, the processor name remains as is.

libs/ardour/ardour/meter.h

@@ -49,6 +49,8 @@ public:
 	void reset ();
 	void reset_max ();
 
+	std::string display_name() const;
+
 	bool can_support_io_configuration (const ChanCount& in, ChanCount& out);
 	bool configure_io (ChanCount in, ChanCount out);
 

libs/ardour/meter.cc

@@ -35,6 +35,8 @@
 #include "ardour/rc_configuration.h"
 #include "ardour/runtime_functions.h"
 
+#include "pbd/i18n.h"
+
 using namespace std;
 
 using namespace ARDOUR;
@@ -73,6 +75,11 @@ PeakMeter::~PeakMeter ()
 	}
 }
 
+std::string
+PeakMeter::display_name() const
+{
+	return _("Meter");
+}
 
 /** Get peaks from @a bufs
  * Input acceptance is lenient - the first n buffers from @a bufs will