bug#48174: meld TypeError on startup

[Nathan Dehnel]

nathan@guixlaptop ~ $ meld
Traceback (most recent call last):
  File "/gnu/store/ry77lgy02rs7605gyn6cai8ff9v64h4l-meld-3.20.3/bin/..meld-real-real", line 385, in <module>
    setup_settings()
  File "/gnu/store/ry77lgy02rs7605gyn6cai8ff9v64h4l-meld-3.20.3/bin/..meld-real-real", line 270, in setup_settings
    meld.settings.create_settings()
  File "/gnu/store/ry77lgy02rs7605gyn6cai8ff9v64h4l-meld-3.20.3/lib/python3.8/site-packages/meld/settings.py", line 98, in create_settings
    meldsettings = MeldSettings()
  File "/gnu/store/ry77lgy02rs7605gyn6cai8ff9v64h4l-meld-3.20.3/lib/python3.8/site-packages/meld/settings.py", line 40, in __init__
    settings.connect('changed', self.on_setting_changed)
TypeError: <Gio.Settings object at 0x7fd6d5157f00 (GSettings at 0x250f7a0)>: unknown signal name: changed