[pgp] Add flatpak manifest and metainfo

flatpak/modules/python3-python-gnupg.yaml

+name: python3-python-gnupg
+buildsystem: simple
+build-commands:
+  - install -d ${FLATPAK_DEST}/lib/python3.7/site-packages/
+  - pip3 install --prefix=${FLATPAK_DEST} python_gnupg-0.4.4-py2.py3-none-any.whl
+sources:
+  - type: file
+    url: https://files.pythonhosted.org/packages/8b/26/14248358136cbd2b24652de2934078d66ed13ae4542c8b0cef0a8fade67d/python_gnupg-0.4.4-py2.py3-none-any.whl
+    sha256: 85c231850a0275c9722f06e34b45a22510b83a6a6e88f93b5ae32ba04c95056c

flatpak/org.gajim.Gajim.Plugin.pgp.metainfo.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<component type="addon">
+  <id>org.gajim.Gajim.Plugin.pgp</id>
+  <extends>org.gajim.Gajim.desktop</extends>
+  <name>PGP Plugin</name>
+  <summary>XMPP Extension Protocol (XEP) for secure multi-client end-to-end encryption</summary>
+  <url type="homepage">https://gajim.org/</url>
+  <metadata_license>CC-BY-SA-3.0</metadata_license>
+  <project_license>GPL-3.0</project_license>
+  <update_contact>gajim-devel_AT_gajim.org</update_contact>
+</component>
+

flatpak/org.gajim.Gajim.Plugin.pgp.yaml

+id: org.gajim.Gajim.Plugin.pgp
+runtime: org.gajim.Gajim
+sdk: org.gnome.Sdk//3.32
+build-extension: true
+separate-locales: false
+appstream-compose: false
+
+build-options:
+  env:
+    PYTHONPATH: /app/plugins/pgp/lib/python3.7/site-packages/
+
+modules:
+  - modules/python3-python-gnupg.yaml
+
+  - name: gajim-pgp
+    buildsystem: simple
+    build-commands:
+      - 'cp -R -t ${FLATPAK_DEST} *'
+    post-install:
+      - install -Dm644 --target-directory=${FLATPAK_DEST}/share/metainfo org.gajim.Gajim.Plugin.pgp.metainfo.xml
+      - appstream-compose --basename=org.gajim.Gajim.Plugin.pgp --prefix=${FLATPAK_DEST} --origin=flatpak org.gajim.Gajim.Plugin.pgp
+    sources:
+      - type: archive
+        url: https://ftp.gajim.org/plugins_releases/pgp_1.2.35.zip
+        sha256: 8227545afdc22102e568fe68fe9837f1b2302030984871f5b57539369b1380cc
+      - type: file
+        path: org.gajim.Gajim.Plugin.pgp.metainfo.xml
+        sha256: cda6e41cdff3f7ff98136721a522305046714343236f9eb54b3e4cba5ccb34ad