Revert "Add CMakeLists.txt.user to the .gitignore file. Closes issue 6873." because no comment has been given on what that newly ignored file even is. If no dev knows what's going on and there's not even a comment explaining things, then that's bad.

This reverts commit 5443f14fe43c56f482af221bc87edfb813c7a429.
Clean up and add info to .gitignore; also re-add the QtCreator CMakeLists.txt.user ignore.
[Android] Stop copying assets that don't need to be copied anymore.