SX: Subcaptionbox in tabu: wrong numbering of subfigures

See https://tex.stackexchange.com/questions/444942/subcaptionbox-in-tabu-wrong-numbering-of-subfigures/

Currently the flags caption, subcaption, and continuedfloat are implemented as boolean values. I guess it would be a better idea to implement them as counters, so packages which prevent global counter changes for a reason (for example the tabu package on the 1st and 2nd run) should be more compatible with the caption resp. subcaption package.