Add a value selection (like for month) for content selectors in custom entry types

[Siedlerchr]

Is your suggestion for improvement related to a problem? Please describe.
When I create content selectors for a field, I would like to choose them from a list of values and not having to remember the value.

Describe the solution you'd like
A value selection like for the month field:

Additional context

1. Create a new library
2. Go to Preferences -> Entry types -> Add a new custom field to an entry type e.g. Misc, for example, name it "mycustom"
3. Save and restart Jabref
4. Right-click on the library-> Library properties -> Content selectors -> Add as in the screenshot below

5. Add an new entry "Misc" into the library 6. See the new custom field 7. you have to type a letter e.g. "k" to get the value suggested

[azatyamanaev]

Hello. Can I work on this issue?

[azatyamanaev]

This issue is still open, so I created a pull request for it.