Add support for specifying main target for CMake projects

When opening a project with multiple executable targets, anyone ends up being the one that is selected by default for running the project. Usually there is a (or multiple) "main" executable(s) that should be selected by default instead.

For qmake projects we have qtc_runnable, for Qbs projects we have qtcRunnable. We should have something similar for CMake.

Since we are limited to what CMake file-api provides us, and option would be to read a cache variable QTC_RUNNABLES with a list of target names.