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