gui: force xcb instead of wayland

[Spaqin]

ARTIQ Pull Request

Description of Changes

Ensures that Wayland is not used, as embedding applets in the GUI is not possible.

Note that the qt.qpa.plugin: Could not find the Qt platform plugin "wayland" in "" warning is still present, even if by default it wouldn't be used (platformName() still returns "xcb").

Related Issue

2513

Type of Changes

	Type
✓	:bug: Bug fix

Steps (Choose relevant, delete irrelevant before submitting)

All Pull Requests

• [x] Use correct spelling and grammar.

Code Changes

• [x] Test your changes or have someone test them. Mention what was tested and how.
• [x] Add and check docstrings and comments

Git Logistics

• [x] Split your contribution into logically separate changes (git rebase --interactive). Merge/squash/fixup commits that just fix or amend previous commits. Remove unintended changes & cleanup. See tutorial.

• [x] Write short & meaningful commit messages. Review each commit for messages (git show). Format:

  topic: description. < 50 characters total.
  
  Longer description. < 70 characters per line

Licensing

See copyright & licensing for more info. ARTIQ files that do not contain a license header are copyrighted by M-Labs Limited and are licensed under LGPLv3+.

[sbourdeauducq]

I don't think we should use QT_QPA_PLATFORM in this way. AFAICT it is only used in debug settings to force a specific platform plugin. On the opposite of what is being done right now, I think we should remove the Wayland check and the -platform xcb if QT_QPA_PLATFORM is set to anything.

[Spaqin]

AFAICT it is only used in debug settings to force a specific platform plugin.

Yes, and we want to force xcb platform plugin; this is more of a guard if the user wanted to. Although probably anyone setting this env variable would know what they're doing.