frontend: Move transition preview button to button box

[exeldro]

Description

Move transition preview button to button box
Before:

After:

Motivation and Context

It is more clear that the button does not belong to the transition properties.
The preview button stays visible while editing the transition.

How Has This Been Tested?

On windows 64 bit by opening the properties of a stinger transition

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.

[WizardCM]

Overall I think this is a good idea, my only concern UX-wise is that it's right next to the Defaults button, which a user might accidentally click instead (though Cancel would undo it, so it's probably safe).

[exeldro]

If people prefer this:

Only have to change the insert position from 1 to 2
The insert position should work as long as there is no help button added according to the order of buttons on:
https://doc.qt.io/qt-6/qdialogbuttonbox.html

[jp9000]

Your proposed change sounds fairly reasonable exeldro. what do you think matt?

[WizardCM]

I'd worry about the non-standard UX, it'd be a bit inconsistent with buttons on other dialogs. Overall though, I'm OK with either option.

[GeorgesStavracas]

@Warchamp7 prefers the bottom-right approach

[exeldro]

Changed to the bottom-right version:

[Warchamp7]

I've rebased this PR

[RytoEX]

Seems fine.

[gxalpha]

Not a huge fan of this approach hacking into the internals of QDialogButtonBox, especially since the layout of the standard buttons can vary between operating systems (and nowhere does Qt guarantee that the QBoxLayout this relies on even exists, that's just an implementation detail and could change at any point).
However QDialogButtonBox::addButton exists, which I think would be a much better solution.

Also, git commit authorship by exeldro should probably be retained, that appears to have been lost in a (manual?) rebase.