use a reasonable number of digits when writing replaygain tags

2 decimals for gains
6 decimals for peaks
(as suggested on http://wiki.hydrogenaud.io/index.php?title=ReplayGain_2.0_specification)

gmusicbrowser_songs.pm

@@ -1266,7 +1266,7 @@ our %timespan_menu=
	type	=> 'float',	check => '#VAL#= do{ #VAL# =~m/^([-+]?\d*\.?\d+)\s*(?:dB)?$/i ? $1 : #novalue#};',
	displayformat	=> '%.2f dB',
	id3v2	=> 'TXXX;replaygain_track_gain;%v',	vorbis	=> 'replaygain_track_gain',	ape	=> 'replaygain_track_gain', ilst => '----replaygain_track_gain',
	prewrite=> sub { length($_[0]) && $_[0]==$_[0] ? $_[0]." dB" : undef }, #remove tag if empty string or NaN
	prewrite=> sub { length($_[0]) && $_[0]==$_[0] ? sprintf("%.2f dB",$_[0]) : undef }, #remove tag if empty string or NaN
	options => 'disable editable',
	category=>'replaygain',
	alias	=> 'track_gain trackgain',
@@ -1277,6 +1277,7 @@ our %timespan_menu=
 replaygain_track_peak=>
 {	name	=> _"Track peak",	width => 60,	flags => 'fgrwscpa',
	id3v2	=> 'TXXX;replaygain_track_peak;%v',	vorbis	=> 'replaygain_track_peak',	ape	=> 'replaygain_track_peak', ilst => '----replaygain_track_peak',
	prewrite=> sub { length($_[0]) && $_[0]==$_[0] ? sprintf("%.6f",$_[0]) : undef }, #remove tag if empty string or NaN
	type	=> 'float',
	options => 'disable',
	category=>'replaygain',
@@ -1289,7 +1290,7 @@ our %timespan_menu=
	type	=> 'float',	check => '#VAL#= do{ #VAL# =~m/^([-+]?\d*\.?\d+)\s*(?:dB)?$/i ? $1 : #novalue#};',
	displayformat	=> '%.2f dB',
	id3v2	=> 'TXXX;replaygain_album_gain;%v',	vorbis	=> 'replaygain_album_gain',	ape	=> 'replaygain_album_gain', ilst => '----replaygain_album_gain',
	prewrite=> sub { length($_[0]) && $_[0]==$_[0] ? $_[0]." dB" : undef }, #remove tag if empty string or NaN
	prewrite=> sub { length($_[0]) && $_[0]==$_[0] ? sprintf("%.2f dB",$_[0]) : undef }, #remove tag if empty string or NaN
	options => 'disable editable',
	category=>'replaygain',
	alias	=> 'album_gain albumgain',
@@ -1301,6 +1302,7 @@ our %timespan_menu=
 replaygain_album_peak=>
 {	name	=> _"Album peak",	width => 60,	flags => 'fgrwscpa',
	id3v2	=> 'TXXX;replaygain_album_peak;%v',	vorbis	=> 'replaygain_album_peak',	ape	=> 'replaygain_album_peak', ilst => '----replaygain_album_peak',
	prewrite=> sub { length($_[0]) && $_[0]==$_[0] ? sprintf("%.6f",$_[0]) : undef }, #remove tag if empty string or NaN
	type	=> 'float',
	options => 'disable',
	category=>'replaygain',