Fix primary clipboard warning

[ConteZero]

Remove warning, should fix #54037

[akien-mga]

Can the getter also trigger errors on platforms where it's not supported, while using LineEdit/TextEdit normally?

[ConteZero]

Yes, you are right, now I have also removed the error message from the getter.
Unfortunately I can only test on Linux so I didn't notice the problem.

[akien-mga]

Looking at the implementation again, I think I was a bit too fast to merge it. The code in LineEdit and TextEdit assumes that the Linux-specific primary clipboard actions are valid, and handles them regardless of that fact.

So that means that hitting middle mouse button on Windows will actually deselect, change caret position, and grab focus - when nothing else happens because it's not a valid action in this environment.

Maybe there should be a DisplayServer::get_singleton()->clipboard_has_primary() that returns whether the DisplayServer supports primary clipboards at all. If not, the events should not be handled. CC @Paulb23

[ConteZero]

Initially the code in line_edit, rich_text_label and text_edit was inside
#ifdef LINUX_ENABLED
...
#endif

then @Paulb23 suggested to remove them.
I could re-add the IFDEF in the code where the getter is called, this way it will not interfere with other platforms behavior.

[Paulb23]

Ah, good point.

DisplayServer already contains a method has_feature(Feature p_feature), should be okay to add FEATURE_CLIPBOARD_PRIMARY to that, we already do something similar for IME support.

[ConteZero]

I've added FEATURE_CLIPBOARD_PRIMARY

[akien-mga]

Aside from my last review comment, this looks good to merge.

[akien-mga]

Thanks!