Make it works with Gedit 3.16.3

[R1kk3r]

Make it works with Gedit of the last Debian testing update. For me, it is Gedit 3.16.3 but it can be compatible with other versions as well (not tested).

Quite similar to pull requests #9 but support internationalization and have a stateful menu entry.

[marc1uk]

On gedit 3.14:
During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/marc/.local/share/gedit/plugins/markdown-preview/init.py", line 25, in
from gi.repository import Gio, Gdk, Gtk, Gedit, GObject, WebKit, GLib
File "/usr/lib/python3/dist-packages/gi/importer.py", line 53, in find_module
'introspection typelib not found' % namespace)
ImportError: cannot import name WebKit, introspection typelib not found

(gedit:6669): libpeas-WARNING **: Error loading plugin 'markdown-preview'

[sixtyfive]

So this PR also suffers from the problem of python-webkit being unmaintained. Perhaps close it, then?

[darkdragon-001]

I just want to let you know, that I created a fork: https://github.com/darkdragon-001/gedit-markdown/

Please test the latest release. Pull requests are always welcome!