Update zlib / add it as subrepo

zlib in \qtbase\src\3rdparty\zlib is currently at version 1.2.5 (from 19 Apr 2010). Since then, there have been a bunch of improvements (see the official ChangeLog) and (according to the author), upgrading is (strongly) advised.

The current version of zlib is: 1.2.11 (from 15 Jan 2017)

As there is an official git repository for zlib on github, why not add that as a subrepository of Qt instead of copy/pasting the library into our repository? This would ease future updates and reduce maintenance work in the future.

And in case it is undesirable to subrepo it directly, just create a new repo containing a subrepo of zlib alongside the .pri files and all the other stuff you need for Qt integration – and add this new repo as subrepo into the Qt repo tree. Both solutions would certainly be cleaner than the current state of affairs

Note: This is a partial duplicate of issue 31020, which seems to have been worked on until it was abandoned somewhere in 2013/2014.

PS.: The current version of zlib creates at least one compiler warning because it tries to left-shift a negative integer (in inflate.c line 1475), which results in undefined behavior according to the C/C++ specs.