Fix Gtk+ warnings

[lene]

https://pastebin.com/fNgSEWqH

[mathiascode]

The tray indicator seems to be causing a few of the common warnings. Does disabling it have any effect for you?

We should replace GtkStatusIcon with AppIndicator3 in any case, since the former is deprecated and doesn't work in Wayland.

[mathiascode]

AppIndicator3 is now in master, so the tray-related warnings are gone at least.

[mathiascode]

Squashed a small warning here: nicotine-plus#223

[mathiascode]

Resolved in nicotine-plus#280