add support for copying arbitrary files

this adds file_copies.prf, which understands the variable COPIES, which
works analogously to INSTALLS.

i've been holding this off for a long time, as it is not without
caveats. however, similar hacks are proliferating all over the code
base, so it's time we formalized it.

in fact, it's the easiest way to fix some nasty shadow build problems,
which is why i'm adding this on the stable branch.

Task-number: QTBUG-52256
Change-Id: Icbe3b9fbb79c952546aad2d467a438d3a69d749f
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>