UI: Add ability for properties to have horizontal layouts

[cg2121]

Description

This adds the ability the properties of objects to have their widgets be placed in a horizontal layout. This extends the obs_property_add_group property.

Example of what it would look like with the video capture source (not included in PR):

Motivation and Context

Idea from @Warchamp7

It makes properties have less vertical spacing.

How Has This Been Tested?

Created layout of buttons in video capture device source.

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.

[cg2121]

Note: the video capture source changes are not included in this PR.

[cg2121]

@Warchamp7 Currently it just squishes everything.

[cg2121]

Updated to include documentation

[GeorgesStavracas]

Latest conclusion: put this on hold until new specs for the sources dialog come up. Reopen if whatever is proposed can be addressed by this approach.