Build plugin: exclude some cmake utility targets

The list of utility targets can quickly become very long, filled with automatically generated targets for automoc, experimental or continuos builds, clang-format, etc. With this patch, only those "utility" targets which are defined directly in the project are included in the build targets list. All executables and libraries are of course also still in the list. So while for some cases some targets will be missing, and the user may have to switch to the command line, I assume for most users this makes this feature much nicer to use.