Bring most mimetypes in line with XDG specifications.  The following are not fully verified in source as of this commit due to ubiquity: html, empty, info, make, man, misc, source, unknown

diff --git a/data/desktop/x-rosegarden-device.desktop b/data/desktop/x-rosegarden-device.desktop
index 4814b0d..a131ed0 100644
--- a/data/desktop/x-rosegarden-device.desktop
+++ b/data/desktop/x-rosegarden-device.desktop
@@ -2,7 +2,7 @@
 Comment=Rosegarden device file
 Comment[fr]=Fichier de description de peripherique Rosegarden
 Hidden=false
-Icon=midi
+Icon=audio-midi
 MimeType=audio/x-rosegarden-device
 Patterns=*.rgd;*.RGD
 Type=MimeType
diff --git a/data/desktop/x-rosegarden21.desktop b/data/desktop/x-rosegarden21.desktop
index 89266b8..4d6f321 100644
--- a/data/desktop/x-rosegarden21.desktop
+++ b/data/desktop/x-rosegarden21.desktop
@@ -2,7 +2,7 @@
 Comment=Rosegarden 2.1 file
 Comment[fr]=Fichier Rosegarden 2.1
 Hidden=false
-Icon=midi
+Icon=audio-midi
 MimeType=audio/x-rosegarden21
 Patterns=*.rose
 Type=MimeType
diff --git a/data/desktop/x-soundfont.desktop b/data/desktop/x-soundfont.desktop
index 03821ad..b39ad25 100644
--- a/data/desktop/x-soundfont.desktop
+++ b/data/desktop/x-soundfont.desktop
@@ -2,7 +2,7 @@
 Comment=Soundfont
 Comment[fr]=Echantillons
 Hidden=false
-Icon=sound
+Icon=audio-x-generic
 MimeType=audio/x-soundfont
 Patterns=*.sf2;*.SF2
 Type=MimeType