35482fa9 breaks offline builds

QWindowKit should get pulled in as a git submodule and not get downloaded while building.
Many packaging tools build inside sandboxes without a network connection and fetch all required sources ahead of time to check them.
Downloading dependencies while building should get absolutely avoided.

Error:

[1/9] Performing download step (git clone) for 'qwindowkit-populate'
Cloning into 'qwindowkit-src'...
fatal: unable to access 'https://github.com/stdware/qwindowkit.git/': Could not resolve host: github.com
Cloning into 'qwindowkit-src'...
fatal: unable to access 'https://github.com/stdware/qwindowkit.git/': Could not resolve host: github.com
Cloning into 'qwindowkit-src'...
fatal: unable to access 'https://github.com/stdware/qwindowkit.git/': Could not resolve host: github.com
-- Had to git clone more than once: 3 times.
CMake Error at qwindowkit-subbuild/qwindowkit-populate-prefix/tmp/qwindowkit-populate-gitclone.cmake:39 (message):
  Failed to clone repository: 'https://github.com/stdware/qwindowkit.git'