Remove MAINTAINERS (does not exist) file from extra dist
Fix EXTRA_DIST for applet gschemas

The following fails to include the needed schemas files:
$(gsettings_SCHEMAS_in_in)
replacing it with:
$(gsettings_SCHEMAS).in.in
Mixer applet moved, correct POTFILES.in
Mixer applet moved so also remove MATE_MixerApplet.server.in
Fix make distcheck

Just use CLEANFILES instead of DISTCLEANFILES
Correct name, server -> service
Add applet_DATA to CLEANFILES