Add note about idle planets with empty build slots

[Xyquas]

Every turn, I go through the turn summary to find planets that have empty build slots. Because every construction is listed there although I have the "suppress minor notes" option enabled I got used to look out for certain key events, such as science building finished (usually this takes the longest time so it is the last one finished).

I'd prefer to have an actual notification about planets that have no more buildings in construction but still have empty building space. I imagine it being like the focus technology maxxed out message in the science part.

So if a planet finishes a civil building and there are no more civil buildings in construction and there is space left for civil buildings generate a notification like "Planet XY has lying dormant areas" (German: "Planet XY hat brachliegende Flächen").