Python bindings issues

I'm trying to use libgranite from python via gir bindings and running into a couple of issues with them.

First, .gir file that's produced during the build is missing a `shared-library=` attribute, as a result you can't instantiate any Granite.* classes:

```
$ python3 -c 'from gi.repository import Granite; Granite.WidgetsSourceList()'
...
TypeError: could not get a reference to type class
```

I reported this at first to Debian ([bug](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=911910), [patch](https://sources.debian.org/patches/granite/5.1.0-2/set-shared-library-attribute.patch/)) as I thought it was distro problem, but the culprit seems to build scripts in this repository and seems to also affect Elementary OS. Please consider cherrypicking their patch.

Second issue I ran into is that overriding `get_context_menu` virtual method for Granite.Widgets.SourceList.Item doesn't work, and as a result I can't create a custom menu for SourceList:

```
#!/usr/bin/env python3
import gi
gi.require_version('Gtk', '3.0')
gi.require_version('Granite', '1.0')
from gi.repository import Granite, Gtk

class Item(Granite.WidgetsSourceListItem):
def __init__(self):
super().__init__(name='item')

def do_get_context_menu(self):
print('do_get_context_menu called')
return Gtk.Menu()

item = Item()
print(item.get_context_menu())
```
Output: None, and my virtual method override (with `do_` prefix) never gets called.

Not sure if the bug is in granite or vala or (py-)gobject. Do you have any ideas how make it work from python?