Port away from deprecated KMessageBox Yes/No API (shim variant)

Using a tempoorary helper namespace PMessageBox (P=Porting) to map code both to old & new API as available, with header-only inline code. Placed into KDev::Utils due to being linked to everything.

Alternative to !379 (closed), with no ifdefelseendifs.

@igorkushnir Not yet exactly sure whether name of file, placement and also shim code itself are proper, so please comment away to get where you see things to end to your satisfaction :)