UI: Disable QDialog button icons on all non-system themes

[tytan652]

Description

Disable icons on QDialog buttons, QStyle like Fusion provide icons to those.

Before:

After:

Motivation and Context

They are ugly and not needed and only Fusion do that. Windows and macOS seems to not have icon for that.

How Has This Been Tested?

Launch a dialog where those icon should show.

Types of changes

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

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.

[jp9000]

This should probably have the "UI: " prefix rather than "themes: " on the commit

[tytan652]

Commit message prefix changed.