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/po/POTFILES.in

@@ -3,4 +3,4 @@
 properties/nm-strongswan.c
 [type: gettext/glade]properties/nm-strongswan-dialog.ui
 auth-dialog/main.c
-NetworkManager-strongswan.appdata.xml.in
+NetworkManager-strongswan.metainfo.xml.in

src/frontends/gnome/po/de.po

@@ -304,27 +304,27 @@ msgstr ""
 "Die Konfiguration verwendet ssh-agent für die Authentisierung, aber ssh-"
 "agent ist nicht gestartet!"
 
-#: ../NetworkManager-strongswan.appdata.xml.in.h:1
+#: ../NetworkManager-strongswan.metainfo.xml.in.h:1
 msgid "strongSwan VPN client"
 msgstr "strongSwan VPN Client"
 
-#: ../NetworkManager-strongswan.appdata.xml.in.h:2
+#: ../NetworkManager-strongswan.metainfo.xml.in.h:2
 msgid "strongSwan based client for IPsec virtual private networks"
 msgstr "Auf strongSwan basierender Client für IPsec VPNs"
 
-#: ../NetworkManager-strongswan.appdata.xml.in.h:3
+#: ../NetworkManager-strongswan.metainfo.xml.in.h:3
 msgid ""
 "Support for configuring IPsec based virtual private network connections."
 msgstr ""
 "Unterstützung für die Konfiguration von IPsec-basierten VPN Verbindungen."
 
-#: ../NetworkManager-strongswan.appdata.xml.in.h:4
+#: ../NetworkManager-strongswan.metainfo.xml.in.h:4
 msgid ""
 "Compatible with IKEv2 key exchange and multiple authentication mechanisms."
 msgstr ""
 "Kompatibel mit dem IKEv2 Schlüsselaustausch-Protokoll und verschiedenen "
 "Authentisierungsmethoden."
 
-#: ../NetworkManager-strongswan.appdata.xml.in.h:5
+#: ../NetworkManager-strongswan.metainfo.xml.in.h:5
 msgid "strongSwan Developers"
 msgstr "strongSwan Entwickler"