UI: Disable qt5ct when compiled with qt6

[kkartaltepe]

Description

Qt is not smart enough to disable qt5 platform plugins when users blindly ask for them, which causes users to be unable to start OBS. Instead lets kindly disable broken platforms on their behalf.

Motivation and Context

Users prefer obs to not crash.

How Has This Been Tested?

It doesnt do anything when I load obs normally. It doesnt fail to load when I set the platform theme.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

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

[tytan652]

My only issue here is that this PR is fixing a situation created by the user themself.

And it is even more complicated since (IIRC) qt6ct also use the name qt5ct along with qt6ct.

[kkartaltepe]

My only issue here is that this PR is fixing a situation created by the user themself.

And it is even more complicated since (IIRC) qt6ct also use the name qt5ct along with qt6ct.

Fair, but its by far the biggest crash source since we switched to qt6 and it is not going away. So it seems better to just prevent users from crashing and provide them our default gui. They can still override this by specify the platform theme on the command line if they "know what they are doing".