CMake - fix bug with dropping changed settings for existing build directory

Summary:
Steps to reproduce:

1) Open some CMake project
2) Create some build directory if it is not created.
3) Open advanced settings ("Show Advanced" button).
4) Change some setting, for example "Build type".
5) Press Ok/Apply.
6) Result - CMake will run with **OLD** settings and all your changes will be lost.

Test Plan: Tested with master branch

Reviewers: #kdevelop, apol

Reviewed By: #kdevelop, apol

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5609

(cherry picked from commit 258f5408)

# Conflicts:
#	projectmanagers/cmake/cmakeutils.cpp