cmake manager: fix incorrect settings import from existing build directory

Summary: Old version ignores install directory and build type settings when user add existing build directory to project. This incorrect behavior leads in particular to full-rebuild if selected build type (which can be choosen from combo box **before** build directory selecting) and existing build type (from build directory) are different. The patch fixes this bug by additional parsing of `CMakeCache.txt` file.

Test Plan: Tested on `master` branch with different projects and build directories.

Reviewers: #kdevelop, apol

Reviewed By: #kdevelop, apol

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2964