Improve pro2cmake so it's usable as an initial step for converting qmake user projects to CMake

The intention is not to advertise it as fool-proof solution for converting qmake user projects to CMake, but rather as a helper that can hopefully handle 60-80% of usual conversion boilerplate.

Off the top of my head some implementation things to consider:

• default to generating public CMake Qt API instead of private API done in <https://codereview.qt&#45;project.org/c/qt/qtbase/<ins>/373517>
• fix qml module generation to use qt_add_qml_module where possible done in <https://codereview.qt&#45;project.org/c/qt/qtbase/</ins>/369502>
• don't generate the INSTALL_EXAMPLEDIR / INSTALL_EXAMPLESDIR stuff - done in <https://codereview.qt&#45;project.org/c/qt/qtbase/<ins>/399125>
• handle required vs optional packages better - done in <https://codereview.qt&#45;project.org/c/qt/qtbase/</ins>/398892>
• generate a single find_package call (or 2 when mixing required + optional) instead of one per line - done in https://codereview.qt-project.org/c/qt/qtbase/+/398826
• see if we can handle plugins to be generated as qt_add_plugins and libs as qt_add_library
• possibly generate qt_finalize_target + MANUAL_FINALIZATION as the default, so that users are aware of the API
• possibly handle translations
• check how subdir projects are handled for user projects - done
  it would be nice if we can detect when pro2cmake is run on a subdir .pro file, so that we don't embed needless calls to cmake_minimum_required, project, CMAKE_INCLUDE_CURRENT_DIR, CMAKE_AUTOMOC, find_package

And some product things to consider:

• do we want to ship it in some form, or just tell people to git clone qtbase to get the script (it's not one file, there are helpers)
• is a blog post enough, or do we want some documentation on the wiki or docs.qt.io
• what parts should be documented? most options were for qt internal usage, some features i would not document or maybe even disable, like the special case preservation mechanism