Give visual clue for cards that can be dragged anywhere

[ByteHamster]

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The cards in the first levels (git init, etc) can be dragged anywhere in the window. When dragging such a card and hovering over a commit, it displays the commit message. That made me think that even "generic" cards that are unrelated to a commit need to be dragged onto a commit. I think it would be better to just highlight the whole main area when a card is dragged there (similar to what is done when dragging a card that can be applied to a specific commit).

Small mock-up how it could look like:

For reference, visual clue when dragging checkout:

[blinry]

I'd really, really like this, as this is confusing playtesters a lot! And I also really like your mockup! A PR would be super welcome, but we might also get around implementing this eventually!