Main Menu: change Library tab

[MartinKarim]

Description of Issue	Possible Solution	Warnings
New article shortcut already exists in toolbar	remove New article from Main Menu	-
Edit strings is an imprecise function name	find a better name (e.g. see prototype)	do not use overly long names
-	-	other changes can be found in issues #4665 , #4666 , #4667

[Siedlerchr]

As I just recently converted the Strings Dialog to javafx, I internally renamed it to BibtexStringEditor. I propose to add at least the word "Bibtex" before so that the user knows what this is about.
Before I touched this dialog I did not really knew what the purpose of the dialog was referring to.