bug#35853: gnome-tweaks unable to manage gnome shell extentions

[Jack Hill <jackhill@jackhill.us>]

I don't know why I didn't think the terminal output said anything useful. 
On a second look, it really does appear to:

```
$ gnome-tweaks

** (..gnome-tweaks-real-real:2439): WARNING **: 22:15:34.941: Failed to load shared library 'libsoup-2.4.so.1' referenced by the typelib: libsoup-2.4.so.1: cannot open shared object file: No such file or directory
WARNING : Error detecting shell
Traceback (most recent call last):
   File "/gnu/store/1fv652flfj50j5w916nggw7d9v6cy1mi-gnome-tweaks-3.28.1/lib/python3.7/site-packages/gtweak/tweaks/tweak_group_shell_extensions.py", line 219, in __init__
     ego = ExtensionsDotGnomeDotOrg(version)
   File "/gnu/store/1fv652flfj50j5w916nggw7d9v6cy1mi-gnome-tweaks-3.28.1/lib/python3.7/site-packages/gtweak/egowrapper.py", line 25, in __init__
     self._session = Soup.Session.new()
GLib.GError: g-invoke-error-quark: Could not locate soup_session_new: libsoup-2.4.so.1: cannot open shared object file: No such file or directory (1)
WARNING : Error detecting shell
Traceback (most recent call last):
   File "/gnu/store/1fv652flfj50j5w916nggw7d9v6cy1mi-gnome-tweaks-3.28.1/lib/python3.7/site-packages/gtweak/tweaks/tweak_group_shell_extensions.py", line 53, in __init__
     self._swInfo = Gio.DesktopAppInfo.new("org.gnome.Software.desktop")
TypeError: constructor returned NULL

(..gnome-tweaks-real-real:2439): Gtk-WARNING **: 22:15:36.303: gtk_window_present_with_time() should not be called with 0, or GDK_CURRENT_TIME as a timestamp, the timestamp should instead be gathered at the time the user initiated the request for the window to be shown
```

In particular, "Error detecting shell" only appears in the 
tweak_group_shell_extensions.py [0].

[0] https://gitlab.gnome.org/GNOME/gnome-tweaks/blob/ee2a669b867894b1f8d58edad861e02ec690c5fe/gtweak/tweaks/tweak_group_shell_extensions.py#L63

I haven't yet looked into why it's hitting those error conditions.

Best,
Jack