Convert Case: exemplify the menus

[mlep]

Is your suggestion for improvement related to a problem? Please describe.
Just trying to make the UI more practical.

Currently, the sub-menus of Change case looks like this:

If you have no background in typography, the meanings are not obvious (Capitalize vs Upper case. Sentence case vs Title case)

Describe the solution you'd like
Alter the sub-menus into:

Capitalize
lower case
Sentence case
Title Case
UPPER CASE

And alter the tips (displayed when you stay long over the menu) accordingly. Example for Title Case:
Capitalize all words, but converts articles, prepositions, and conjunctions to lower case.
will become
Capitalize all Words, but Converts Articles, Prepositions, and Conjunctions to Lower Case.

Tip: change JabRef UI into French language to looks what it will look like (yeah, this is already implemented in French)

[koppor]

This is the screenshot for the menu location:

French:

[manv-afk]

Hello, I've made a new PR for this issue, don't hesitate to give me some feedback if I need to change anything else for this issue.