Allow captions also on non-floating minted listings

Minted provides for captions only with floating listings. However,
listings always allows captions, and our machinery is geared accordingly.
So, instead of discriminating the floating and non-floating cases,
always allow for captions with minted, too. When minted does not provide
for a caption, we will provide one.
In the child document case the caption will always be before the listing,
while for the inset case the caption will be before the listing if it
is entered as the first line, after the listing otherwise.