[D10?][META] Backport any dropbutton improvements that apply to Backdrop

[klonos]

Add new Splitbutton render element to eventually replace Dropbutton lists some Drupal core issues related to improvements/changes that have been implemented over time, that may apply to us as well. Creating this issue as a placeholder/reminder to go through those issues, and see if there's anything that we could benefit from.

• #1608878: Add CTools dropbutton to core
• #1799498: Improve dropbutton
• #1751606: Move published status checkbox next to "Save"
• #3168326: Dropbuttons in table cells are potentially unusable if it includes options with long text length.
• #2278479: Change the dropbutton theme function to ensure consistent output and accommodate the splitbutton design

The issue summary also mentions the following 2 problems:

• The open/closed state of the menu isn't conveyed to assistive technology, it is only apparent visually.
• The current focus style for the more-actions button is weak, and won't pass WCAG 1.4.11 Non-text contrast.

The proposal of that issue is to introduce a different element, which may or may not be appropriate for us. We should look closer into that as well.