Use v2 of obs_properties_add_button

[gxalpha]

Description

This makes it explicit what the data pointer actually is.

Motivation and Context

I want to deprecate v1 of obs_properties_add_button, see obsproject/obs-studio#12528.

How Has This Been Tested?

macOS 26
Tested that clicking the button still correctly reloads the page.

Types of changes

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

• 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.