[Development] Switch the main "Qt Build System"

[Alexandru Croitor]

Hi everyone,

It's time to talk about everyone's favourite topic again: build systems. 

It's been a while since we started porting and merging the CMake ports of Qt repositories into the dev branches.

To be precise: qtbase got merged on the 7th of February.

Since then, all the enabled repositories in qt5.git/dev have CMake ports merged to their corresponding dev branches (except for qtqa which got reverted, but it should be back soon).

The CMake ports are built in Coin with the most important configurations (Linux, Windows, macOS, Android, iOS, qemu Linux).

Currently, failed tests don't fail integrations, but we're fixing up the last failing tests and intend to enforce successful test passing for CMake configurations.

Anyway, it's time for the next step.


-----------------------------> TL;DR here <----------------------------------------

The CMake Port team wants to switch the main Qt build system to the CMake one.

What this means:

- All Coin qmake configurations in dev branch are removed.

- Only CMake configurations are built in Coin.

- CMakeLists.txt files are now the source of truth (no more editing of .pro / .pri files).

- All qmake .pro / .pri files of ported repositories under Coin control are removed.

- No more pro2cmake.py and configurejson2cmake.py usage (except for an initial port of a yet un-ported repository)

- You need CMake to build Qt.

- Un-ported repositories should still be able to build with qmake against a CMake built Qt (this was tested to work for qtwebengine, but not other repositories, there are a few known issues left that we're currently fixing).


--> Proposed time of execution: 1st of July <--


I'd like to hear about any Qt development blockers that you foresee due to this change.


Regards,
Alexandru.