flatpak (build_manifest): updated org.freedesktop.Platform runtime version: manifests were using an old 1.6 version of the runtime, this patch fixes them by using the current 18.8 one. Thanks to that, the main_loop.patch which was made because of an outdated version of GLib (see https://github.com/flatpak/flatpak/issues/1899) is not needed anymore and has been removed. Some useless extensions have also been removed from default manifest.

[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;