Revert "Remove maintainership"

I'm baaaaack!

This patch reverts my patch where I removed myself as maintainer because
of my traveling. I'm back now and I want to maintain these packages
again.

This reverts commit ce1c1e3093.

pkgs/desktops/xfce/panel-plugins/xfce4-mailwatch-plugin.nix

@@ -21,6 +21,6 @@ stdenv.mkDerivation rec {
     homepage = "http://goodies.xfce.org/projects/panel-plugins/${p_name}";
     description = "Mailwatch plugin for Xfce panel";
     platforms = platforms.linux;
-    maintainers = [ ];
+    maintainers = [ maintainers.matthiasbeyer ];
   };
 }

pkgs/desktops/xfce/panel-plugins/xfce4-mpc-plugin.nix

@@ -21,6 +21,6 @@ stdenv.mkDerivation rec {
     homepage = "http://goodies.xfce.org/projects/panel-plugins/${p_name}";
     description = "MPD plugin for Xfce panel";
     platforms = platforms.linux;
-    maintainers = [ ];
+    maintainers = [ maintainers.matthiasbeyer ];
   };
 }

pkgs/desktops/xfce/panel-plugins/xfce4-timer-plugin.nix

@@ -25,6 +25,6 @@ stdenv.mkDerivation rec {
     description = "A simple XFCE panel plugin that lets the user run an alarm at a specified time or at the end of a specified countdown period";
     platforms = platforms.linux;
     license = licenses.gpl2;
-    maintainers = [ ];
+    maintainers = [ maintainers.matthiasbeyer ];
   };
 }