Add support for startup project(s)

[njannink]

Support for definition of the start-up project/projects in the slnx file would be very helpfull.

[Mrxx99]

I think having the option to define the default startup project(s) is more useful than to sync it whenever it is changed by a user in VS.

[njannink]

Yes makes sense to only save the default startup project(s) and keep the user defaults in the .vs folder