Implement a dropdownParent feature

[zoglo]

Description

The implementation would allow specifying a dropdownParent and if provided, all dropdowns would be appended to it. Additionally, each dropdown is still positioned absolute to the parent (I scratched the idea of position: fixed in favor of following the KISS principle and positioning calculation).

This would implement and fix the following:

• dropdown visibility height #930
• Behaviour of dropout elements on mobile device #1131
• Dropdown parent must be configurable to avoid overflows #1288
• Dropdown - Body appendTo option #1309
• What's the equivalent to Select2's DropdownParent: 'body in Choices? #1340

This isn't a feature I need right now (not in the next release and I don't really want to delay that one 🙈), but I experimented with implementing it. It still requires significant selector adjustments for the CSS to work correctly (currently, dropdown styles aren’t fully applied).

Maybe someone else might want to build on this functionality in the future. This PR could serve as a head start and provide some documentation for that purpose.

Screenshots (if appropriate)

Types of changes

• Chore (tooling change or documentation change)
• Refactor (non-breaking change which maintains existing functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• My code follows the code style of this project.
• I have added new tests for the bug I fixed/the new feature I added.
• I have modified existing tests for the bug I fixed/the new feature I added.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.