SetGroupIDLayer Mixed Values improvements

[Fryy55]

This PR consists of 2 main changes:

1. I’ve changed the way “Next Free” buttons are handled in mixed value inputs. Now they store a pointer to SetGroupIDLayer’s member callback function, and create the button if a callback is present. This helps maintain compatibility with other mods, as well as reduces boilerplate. Another change to this system is the “Next Free” button’s offset - for trigger order this button should be located to the left of the input field, which is handled with the offset. The “Next Free ORD” button not appearing is also fixed with these changes.
2. MixedValuesConfig now has the placeholder text field, which, if present, replaces the default “Num” placeholder text of the input field and makes going to 0 on the input field show the placeholder. This is necessary for CH and ORD input fields and is a vanilla feature, because those input fields lack description labels and are hard to tell apart without them.

Hope you find these changes useful and worth merging.

Best regards,
Fryy_55 💜

[Fryy55]

ok idk how to add issues to the sidebar but it fixes #644

[HJfod]

Minor issues that make this unable to be merged yet, but overall looks good! Thank you very much!