Provide project selector to pick startup project

[DustinCampbell]

Picking a new startup project, should modify the launch.json so that F5 launches the project output under the debugger.

[gregg-miskelly]

Here is an idea -- instead of asking the user which project they want as the startup project (or maybe in addition to asking), maybe we should generate a different entry in launch.json and tasks.json for each launchable project. That way the user can easily switch around the startup project using the drop down built into VS Code.