Fixed bugs with MultiDialog. Fixed Bug with prompting about modules.

Previously MultiDialog attempted to centre on parent dialog, but there was a NULL parent.  Now the application top window is used. Dialog shows centred, for example when there are missing block files.  If the top window is a WX_STAY_ON_TOP, we move our dialog to the left, as otherwise it would be partially hidden.

Previously Audacity would ask about enabling a module whilst the splash screen was showing.  For some as yet undetermined reason the MultiDialog and Splash Screen are incompatible.  Possibly it's related to doing a ShowModal before a full application exists.  The not ideal workaround is to now delay showing the splash screen until after the modules have been loaded.

src/prefs/ModulePrefs.cpp

@@ -47,6 +47,8 @@ void ModulePrefs::GetAllModuleStatuses(){
    //    mod-script-pipe
    //    mod-nyq-bench
    //    mod-track-panel
+   //    mod-menu-munger
+   //    mod-theming
 
    // TODO: On an Audacity upgrade we should (?) actually untick modules.
    // The old modules might be still around, and we do not want to use them.