Re-add support for Gnome 3.28

[MageJohn]

Hey, I see you've dropped support for Gnome versions before 3.30; I get you don't want to support too many versions, but could you consider adding support back for Gnome 3.28? I use this extension on systems that have Ubuntu 18.04 LTS, and 3.28 is the only version of Gnome available. Ubuntu 18.04 (with the default Gnome desktop) is going to be around a while, it would be a pity if none of them could use up to date versions of this extension!

As a note to anyone still stuck on 3.28, the last working commit, as far as I can tell, is d767cfeb2c7613542e92ae0001e9e6048085a66e