Run under flatpak

Tentative manifest to run under flatpak.

To build:

    flatpak-builder --ccache --force-clean build org.gnome.Gtg.json

To run:

    flatpak-builder --run build org.gnome.Gtg.json gtg

Added file:

org.gnome.Gtg.json

```
{
  "app-id": "org.gnome.Gtg",
  "runtime": "org.gnome.Platform",
  "runtime-version": "3.28",
  "sdk": "org.gnome.Sdk",
  "command": "gtg",
  "cleanup": [
    "/include",
    "/lib/pkgconfig",
    "/share/pkgconfig",
    "/share/aclocal",
    "/man",
    "/share/man",
    "/share/gtk-doc",
    "/share/vala",
    "*.la",
    "*.a",
    "*.pyc",
    "*.pyo"
  ],
  "build-options": {
    "cflags": "-O2 -g",
    "cxxflags": "-O2 -g",
    "env": {
      "V": "1"
    }
  },
  "finish-args": [
    "--share=ipc",
    "--socket=x11",
    "--socket=wayland",
    "--filesystem=host",
    "--talk-name=ca.desrt.dconf",
    "--env=DCONF_USER_CONFIG_DIR=.config/dconf"
  ],
  "modules": [
    {
      "name": "python-dbus",
      "sources": [
        {
          "type": "archive",
          "url": "https://pypi.python.org/packages/ad/1b/76adc363212c642cabbf9329457a918308c0b9b5d38ce04d541a67255174/dbus-python-1.2.4.tar.gz",
          "sha256": "e2f1d6871f74fba23652e51d10873e54f71adab0525833c19bad9e99b1b2f9cc"
        }
      ],
      "build-options": {
        "env": {
          "PYTHON": "python3.5"
        }
      }
    },
    {
      "name": "liblarch",
      "no-autogen": true,
      "buildsystem": "simple",
      "build-commands": [
        "python3 setup.py build",
        "python3 setup.py install --prefix /app"
      ],
      "sources": [
        {
          "type": "git",
          "url": "https://github.com/getting-things-gnome/liblarch"
        }
      ]
    },
    {
      "name": "pyxdg",
      "no-autogen": true,
      "buildsystem": "simple",
      "build-commands": [
        "python3 setup.py build",
        "python3 setup.py install --prefix /app"
      ],
      "sources": [
        {
          "type": "git",
          "url": "https://gitlab.freedesktop.org/xdg/pyxdg"
        }
      ],
      "build-options": {
        "env": {
          "PYTHON": "python3.5"
        }
      }
    },
    {
      "name": "gtg",
      "no-autogen": true,
      "buildsystem": "simple",
      "build-commands": [
        "python3 setup.py build",
        "python3 setup.py install --prefix /app"
      ],
      "sources": [
        {
          "type": "git",
          "url": "https://github.com/getting-things-gnome/gtg"
        }
      ]
    }
  ]
}
```

it fails with:

```
$ flatpak-builder --run build org.gnome.Gtg.json gtg
/app/bin/gtg:54: PyGIWarning: Gdk was imported without specifying a version first. Use gi.require_version('Gdk', '3.0') before import to ensure that the right version gets loaded.
  from gi.repository.Gdk import Screen

(gtg:3): dbind-WARNING **: 08:58:10.314: Couldn't connect to accessibility bus: Failed to connect to socket /tmp/dbus-W9FZd9CTBT: Connection refused
/app/lib/python3.5/site-packages/GTG/gtk/manager.py:22: PyGIWarning: Gtk was imported without specifying a version first. Use gi.require_version('Gtk', '3.0') before import to ensure that the right version gets loaded.
  from gi.repository import GObject, Gtk
Traceback (most recent call last):
  File "<frozen importlib._bootstrap>", line 890, in _find_spec
AttributeError: 'DynamicImporter' object has no attribute 'find_spec'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/app/bin/gtg", line 62, in <module>
    from GTG.gtk.manager import Manager
  File "/app/lib/python3.5/site-packages/GTG/gtk/manager.py", line 26, in <module>
    from GTG.gtk.browser.browser import TaskBrowser
  File "/app/lib/python3.5/site-packages/GTG/gtk/browser/browser.py", line 34, in <module>
    from GTG.gtk.browser.custominfobar import CustomInfoBar
  File "/app/lib/python3.5/site-packages/GTG/gtk/browser/custominfobar.py", line 26, in <module>
    from GTG.tools.networkmanager import is_connection_up
  File "/app/lib/python3.5/site-packages/GTG/tools/networkmanager.py", line 23, in <module>
    from gi.repository import NetworkManager, NMClient
  File "/usr/lib/python3.5/site-packages/gi/importer.py", line 127, in find_module
    'introspection typelib not found' % namespace)
ImportError: cannot import name NetworkManager, introspection typelib not found
```

fork in https://github.com/albfan/gtg/tree/flatpak

issue: https://github.com/albfan/gtg/issues/1