This patch fixes wrong CMake arguments during adding new build directory by user.

Reproducible - always. Steps to reproduce:

1. Open some CMake-based project and go to it's configuration.
2. Open CMake settings.
3. Press "Show Advanced" button.
4. Press button to add new build directory.
5. Change some parameters, for example build type to MinSizeRel.
6. Press "OK".
7. Selected parameters will be "dropped" and replaced by it's default values. For example, build type will have Release value.

As a result, user can't set any CMake parameters for new build directory - all new directories will be Release builds with default settings.

This caused by wrong order in the CMakePreferences::createBuildDir() method: it first adds new directory into combo box and then save parameters values to config. But adding directory to the combo box leads to calling CMakePreferences::buildDirChanged() method, which will try to read not yet populated config.

Tested on master branch.