Place build generated files in separate folder

[ann0see]

Describe the bug

Currently, the build generated files e.g. .moc are scattered in the whole root directory. This is the case especially on manual builds.
#2588 tried to clean them up by moving them into a separate folder however it was not successful.

This is annoying as there are a lot of files.

Please note that if the files are moved, the .gitignore file needs to be updated too.

To Reproduce

Run a build on e.g. Linux and see the files in the root.

Expected behavior

Have a clean root directory.

Questions/Comments

• Why is this a problem? I think overview is the main issue
• Cross compilation is more difficult
• Make clean removes them

[pljones]

QtCreator does out-of-tree builds by default.

make distclean; qmake && make && make clean (with any qmake / make options) keeps the project root clean and leaves the user with the option of not running make clean.

make needs to remain re-runnable from a point of failure with any proposed solution.