Jabref: Entry Editor: general changes

[MartinKarim]

|Description of Issue|Possible Solution| Warnings |
|:-----------------------------------------:|:-------------------------------------------------:|:---------------:|
| Text fields are empty per default;
there is no guide what to write in a field and in what form (e.g. text, numbers)|text suggestions in grey for every text field| font color needs to be very different from normal text;
do not use suggestions if the form of input can vary;|
| flat buttons at the end of text fields are too inconspicuous;
button icons are not always recognized|make buttons bigger and elevated;
use text buttons, sometimes in combination with icons|-|
|too many tabs in the entry editor;
some information repeats in other tabs;
information structure does not match the users expectation;|remove tabs from the entry editor;
divide fields on new tabs with more fitting structure;|-|
|||changes to the specific tabs can be found in #4687 #4688 #4689 #4690 #4691 |

[LinusDietz]

I like the change with the more explicit button. I would prefer if it would shorten the field instead of being in a separate column. (here the prototype is maybe a bit misleading)

[koppor]

This is a minimal change. We think of doing some more UI changes.