flatpak: set appdata release version for dev version: appdata file can now use a template (which has the same name with "_tpl_" prefix), which is used in dev version to automatically set the release with the right Mercurial revision/date. Comment from org.salutatoi.Cagou.appdata.xml has been moved as first child of <component>, so it is not lost while parsing the file.

[Desktop Entry]
Type=Application
Name=Cagou (SàT)
Comment=desktop/mobile frontend for Salut à Toi (XMPP)
Exec=sat_wrapper
Icon=org.salutatoi.Cagou
Categories=Network;InstantMessaging;Chat;FileTransfer;RemoteAccess;
Keywords=xmpp;chat;messaging;im;file;sharing;communication;jabber;social;network;decentralization;desktop;