textproc/zxing-cpp: update to 1.2.0

There have been nearly 800 commits that touched about 10k lines of code
since the last release.

Here are the highlights/major changes:

• switch to C++-17 (client code can still be compiled with a C++-11 compiler)
• switch to the reimplemented 1D detectors (about 5x faster than old ones in false-positive use case)
• deprecate the use of ResultMetadata, see ZXingReader.cpp as an example
• completely new Android wrapper library with CameraX support, integrated build system and demo app
• a lot faster and more capable isPure detection for all 2D codes
• completely new QRCode reader (faster, better support for rotated symbols)
• new BarcodeFormat names (old ones still available but deprecated)
• new ZXingQtCamReader demo app based on QtMultimedia and QtQuick
• reduced false positive rate for UPC/EAN barcodes + improved Add-On symbol handling
• 20% faster ReedSolomon error correcting
• PDF417 is faster and supports flipped symbols
• DataMatrix: added DMRE support
• much improved python wrapper incl. PyPI module for easy pip installation (thanks to antoinehumbert)
• proper ECI handling in all 2D barcodes (thanks to gitlost)
• added Structured Append support for DataMatrix, Aztec and MaxiCode (thanks to gitlost)

Note: The next release will drop support for a number of deprecated
APIs. Please update your code and fix all deprecation warnings now.