Combine SearchableComboBox and TextField for adding fields in Custom Entry Types

[Memeel]

I previously worked on an issue, which has been resolved and merged (#14189).

During the implementation of that fix, I realized that the current user interface for adding fields could be optimized.
Currently, the distinction or separation between selecting a standard field (via a SearchableComboBox) and entering a custom field name (via a free text field) can be removed.

I have recently revisited this topic and found a way to combine these two inputs into a single control. The solution replaces the separate boxes with a unified TextField that supports auto-completion. This allows users to:

• Select a standard field from the suggestion list
• Type a custom field name freely if it doesn't exist in the list

I have already successfully implemented this solution locally and would appreciate feedback on whether this modification is considered interesting for the project.

Additional context: This is a follow-up to the work done in #14082.

[Siedlerchr]

@Memeel That sounds great! This could. be really useful for other parts in JabRef as well