This cleans up the duplicate methods and operations that resulted from the recent merge of Morten's cmake executable preferences patch to the main branch with my own to the 5.2 branch.
This patch was made against the master branch.
I've standardised on currentCMakeExecutable() for consistency with existing conventions and stuck to my approach that centralises the fallback from project-setting to kdevelop-setting to hardcoded system default.
I've kept Morten's simpler use of a QString& argument in the setter, though.