nm: Migrate appdata to metainfo

The path '/usr/share/appdata' is deprecated as is the .appdata.xml
extension, files should be in installed in '/usr/share/metainfo' with
a .metainfo.xml extension.

According to the docs, the metainfo path should be well supported even
by older distros like Ubuntu 16.04.

Reference: 2.1.2. Filesystem locations
https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html

src/frontends/gnome/Makefile.am

@@ -6,9 +6,9 @@ nmvpnservice_DATA = nm-strongswan-service.name
 
 @INTLTOOL_DESKTOP_RULE@
 
-appdatadir = $(datadir)/appdata
-appdata_DATA = $(appdata_in_files:.xml.in=.xml)
-appdata_in_files = NetworkManager-strongswan.appdata.xml.in
+metainfodir = $(datadir)/metainfo
+metainfo_DATA = $(metainfo_in_files:.xml.in=.xml)
+metainfo_in_files = NetworkManager-strongswan.metainfo.xml.in
 @INTLTOOL_XML_RULE@
 
 if WITH_LIBNM_GLIB
@@ -37,12 +37,12 @@ nm-strongswan-service.name: $(srcdir)/nm-strongswan-service.name.in
 
 EXTRA_DIST = \
     nm-strongswan-service.name.in  \
-    $(appdata_in_files)  \
+    $(metainfo_in_files)  \
     intltool-extract.in  \
     intltool-merge.in    \
     intltool-update.in
 
-CLEANFILES = $(nmvpnservice_DATA) $(appdata_DATA) *~
+CLEANFILES = $(nmvpnservice_DATA) $(metainfo_DATA) *~
 DISTCLEANFILES = intltool-extract intltool-merge intltool-update
 
 ACLOCAL_AMFLAGS = -I m4