Added ProgramWidget. LiveProgramWidget now uses it. Refactored, cleaned up.

LiveProgramWidget has a clean starting state (like LiveGofmtWidget)
instead of loading GenProgram.go.
Report error when `go build` doesn't produce output file.
Moved Project::SomethingFromAppRenderTEST() to
ProgramWidget::ProcessTime().
Project::GetSourceOnChange() is no longer used by latest code.
GR shortcuts can have nullptr action (instead of crashing).
LiveEditorApp, LiveFunctionWidget, Go Autocompletion disabled for now,
need to re-enable.