Added Button to restore recently deleted beatmaps

[FreezyLemon]

Closes #1215.
This now works reliably after the problem mentioned in that issue has been fixed.
However the BeatmapCarousel will not update when the button is clicked while having SongSelect open, though this should be fixed by #1657 .

[FreezyLemon]

Update: There was some misunderstanding here. I didn't notice at first, but the map(set) actually does get added again (the carousel updates correctly), but it gets added at the bottom which confused me a bit since I have a lot of maps installed.
Maybe this needs to be changed to re-sort when adding new maps aswell?

[Aergwyn]

After looking into this why they appear at the end I noticed something.

It does restore circles! too, which (I assume) shouldn't happen. Even if I'd love to play it.

[FreezyLemon]

Fixed that oversight. The beatmap hash for the "menu music"'s beatmap is now inside BeatmapManager to centralise it a bit (and since I needed it twice I thought it would make more sense to put it there).

[peppy]

Can't you just check the Protected flag and skip it? That's why it's there.

[peppy]

As this is a non-instant operation, I think a progress notification should be added similar to the other tasks associated with these buttons.

[FreezyLemon]

It would be nice to have #1707 merged (or closed, for that matter) before this so I can add a custom CompletionText without re-PRing.

[peppy]

#1707 has been merged now – wanna update this to take advantage of it?

[peppy]

@FreezyLemon make sure to pay attention to that change I made. quite important from a performance perspective (every Count() and Any() and foreach was hitting the database context to re-query in your previous implementation).