Fix MixerBoard Theming

Fixed so that updates to theme are applied immediately.  Previously theming only worked properly after a restart with the new theme.  Paul found that you could create a Chimera MixerBoard with a track in each of the four different themes.

I chose to completely recreate the MixerBoard on a prefs update, rather than the more fiddly detail of retheming each component of it.

src/Project.h

@@ -562,7 +562,9 @@ public:
  public:
    void ModifyState(bool bWantsAutoSave);    // if true, writes auto-save file. Should set only if you really want the state change restored after
                                              // a crash, as it can take many seconds for large (eg. 10 track-hours) projects
-private:
+   void RecreateMixerBoard();
+
+ private:
    void PopState(const UndoState &state);
 
    void UpdateLyrics();
@@ -653,8 +655,8 @@ private:
 
    HistoryWindow *mHistoryWindow{};
    LyricsWindow* mLyricsWindow{};
-   MixerBoard* mMixerBoard{};
    MixerBoardFrame* mMixerBoardFrame{};
+   MixerBoard* mMixerBoard{};
 
    Destroy_ptr<FreqWindow> mFreqWindow;
    Destroy_ptr<ContrastDialog> mContrastDialog;