Split Inits between OnInit() and FinishInits()

This is after reading a note in the release notes for wxWidgets 3.0.0.  We need an event loop running to show a dialog in 3.0.0 (even though the splash screen works without).

src/AudacityApp.h

@@ -96,6 +96,10 @@ class BlockFile;
 class AudacityApp:public wxApp {
  public:
    virtual bool OnInit(void);
+   void FinishInits();
+#if wxCHECK_VERSION(3, 0, 0)
+   virtual void OnEventLoopEnter(wxEventLoopBase * pLoop);
+#endif
    virtual int OnExit(void);
    virtual void OnFatalException();