frontend: Enforce Qt to have a DE-agnostic behavior on Linux

[tytan652]

Description

Enforce Qt to have a DE-agnostic behavior on Linux

I didn't have Plasma screenshot since I have no machine under it.

Before (with Qt 6.8 on GNOME):

After:

Motivation and Context

If the platform theme is not enforced, Plasma Wayland will enforce SSD decoration (that also lead to some menu issues depending on the distro defaults). This also has made some users think that we explicitly support SSD which is not the case, we rely on Qt default CSD.

If the wayland decoration is not enforced, under GNOME, Qt (since 6.8) will try to enforce a GNOME-like decoration. DO NOT mix it up with GNOME X11 SSD decoration.

There is also third-party Qt plugins (e.g. qt5ct, qgnome) that caused their share of issues in the past, removing them from the list of potential culprit still can be useful.

I think this is not controversial to expect OBS Studio to look the same under any desktop environment.

How Has This Been Tested?

Ran locally OBS (outside of a Flatpak) and no issue were met using file chooser (use portals now rather the GTK 3 dialog) and the window decoration is Bradient.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.