Consider creating a qt.conf file in the build dir of an app when using a static relocatable Qt

Currently the relocatable feature is disabled by default when configuring a static Qt.

When enabled, that means that running applications from the build directory will not find qt provided translations and qml modules.

See
https://codereview.qt-project.org/c/qt/qtbase/+/210936/30#message-f3fd1bac43f08fe924fb3a0307942e43781e779c
and
https://codereview.qt-project.org/c/qt/qtbase/+/210936/30#message-82c9151682f5ed1baf6132569687009692378f60

What we could consider doing is creating a qt.conf inside the app build dir.
It would be created by a qt_add_executable() CMake call.
The qt.conf would have translations and qml imports entries, as absolute paths pointing to the Qt installation dir.
We would only do it for static + relocatable case, to reduce potential breakage.

We should ensure that the qt.conf is not installed. We should also provide an opt-out, which would be useful in case if the project already creates a qt.conf file.

Creating the qt.conf automatically will be a small breakage for projects that already create it themselves, but the scope is hopefully rather small (static + relocatable).