UI: Give private spacing helper sources names

[gxalpha]

Description

Gives names to the spacing helper label private text sources in the preview, to make the log a bit nicer.

If people think the dstr is overkill (I'm a bit torn on that) I'd also be happy to change it to a generic Preview spacing label instead.

Motivation and Context

When you click on a scene item for the first time in a program run, you get this log:

debug: private source '(null)' (text_ft2_source) created
debug: private source '(null)' (text_ft2_source) created
debug: private source '(null)' (text_ft2_source) created
debug: private source '(null)' (text_ft2_source) created

While this obviously isn't a huge issue I think it's nicer to have debug lines that actually say something, so with this PR you get this:

debug: private source 'Preview spacing label 0' (text_ft2_source) created
debug: private source 'Preview spacing label 1' (text_ft2_source) created
debug: private source 'Preview spacing label 2' (text_ft2_source) created
debug: private source 'Preview spacing label 3' (text_ft2_source) created

How Has This Been Tested?

macOS 14.4.1
Ran OBS, clicked a scene item. Observed the log above.
Also created a normal source called Preview spacing label 0 to be 100% sure that the private source names don't cause collisions.

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.

[Lain-B]

Other than the feedback above I generally approve of this

[norihiro]

It looks good to me.