UI: Enforce completely Fusion Qt style on Linux

[tytan652]

Description

As of 8dcfae9, indicating the base style to the proxy is completely functional.

It also bypasses QT_STYLE_OVERRIDE and -style, but since the system theme is no longer available on Linux this is a non-issue.

Motivation and Context

Rewrite of #6664 to use the conventional method to enforce a Qt style.

Since #10602, issues with the conventional method were fixed since there is one style for the whole app.

Also #9653 phased out the system theme on Linux and other themes only support Fusion as a base style on Linux.

So the side-effect of #6664 allowing the user to set their own Qt style is mostly unusable and potentially unwanted.

How Has This Been Tested?

Enforced system theme in the config file, and try to ran OBS with Breeze enforced.
With the PR, Fusion was in use and not Breeze.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

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.