enhance "edit name" dialog

[r10s]

in the "edit contact name" dialog (accessible from "contact profile"), it is a bit unclear, how "Name" us used:

we prepared the following strings for enhancement (this requires the strings to be updated first):

• use string edit_name_placeholder as a placeholder for the input field ("test" in the screenshot)
• use edit_name_explain as text below or above the input field
• both strings take the result of dc_contact_get_auth_name() as a parameter. if empty, use dc_contact_get_addr(). do not use the "display name" ¹

the email address and layout can stay as is.

cave: existing strings and placeholders should still be used when the dialog is used as "add contact manually" for non-chatmail accounts

compare deltachat/deltachat-desktop#4090 and deltachat/deltachat-android#3254

Footnotes

1. the "display name" may be the nickname, this is fine nearly everywhere else and indeed the preferred method to display a name - but not in this special case :) ↩