apprt/gtk: remove explicit X11 clipboard atom

[vancluever]

Fixes #9532
Supersedes #9554

Turns out the explicit UTF8_STRING atom was not needed after all and GTK adds it automatically when running under X11; just having the explicit UTF-8 charset type is enough.

This corrects situations where it may not be necessary to include (Wayland), in addition to removing a duplicate atom under X11.

Importantly, this also corrects issues under Wayland in some scenarios, such as using Electron-based apps (e.g., VSCode/Codium under Ubuntu 24.04 LTS).

[mitchellh]

Thank you so much for the diligence on this!

[vancluever]

No problem @mitchellh!

Another small addendum: I just decided to check foot, and they offer UTF-8 first, along with the X11 atoms (offered last). However, pretty sure my assumption about the actual selected type being up to the client still stands:

(https://wayland.freedesktop.org/docs/html/ch04.html#sect-Protocol-data-sharing)

Data negotiation

A client providing data to other clients will create a wl_data_source object and advertise the mime types for the formats it supports for that data through the wl_data_source.offer request. On the receiving end, the data offer object will generate one wl_data_offer.offer event for each supported mime type.

The actual data transfer happens when the receiving client sends a wl_data_offer.receive request. This request takes a mime type and a file descriptor as arguments. This request will generate a wl_data_source.send event on the sending client with the same arguments, and the latter client is expected to write its data to the given file descriptor using the chosen mime type.

So we got some extra context here in the event that we see more clients misbehave than not and we actually do need to adjust the order (or actually add the X11 atoms in Wayland).