Main Menu: disable unavailable items

[MartinKarim]

Description of Issue	Possible Solution	Warnings
Unavailable items in the Main Menu are greyed out and disabled when no .bib file is open. If a .bib file is open, items that can still not be used (e.g. because it is necessary to have entries selected first) are not disabled anymore	Gray out and disable items, that are unavailable, in all Main Menu tabs	consider all conditions for using the function of that item (e.g. all entries selected vs one entry selected vs multiple selected)

[AliZDev-v0]

I these issue still open?

[tobiasdiez]

It's still not fixed but also a bit more complicated. See #4795 for details.

Duplicate of #4795.