Add wxPreferencesPage::GetIcon() returning wxBitmapBundle

Allow returning a wxBitmapBundle rather than an individual wxBitmap.

Also make GetLargeIcon() non-pure even in wxOSX, as it now doesn't need
to be overridden if GetIcon() is -- but don't make GetIcon() pure
virtual neither to allow the existing code overriding GetLargeIcon() to
keep working.

This incidentally fixes the icons for the standard pages under macOS
broken by 388d322b68 (carry changes to toolbar over to prefs on osx,
2021-09-28), which replaced return statements with assignments,
resulting in the icon being set to the last value assigned to it instead
of the correct one -- this commit restores the previous control flow in
wxStockPreferencesPage::GetLargeIcon() (now called GetIcon()).

Closes #22187.

interface/wx/preferences.h

@@ -147,15 +147,21 @@ public:
     virtual wxString GetName() const = 0;
 
     /**
-        Return 32x32 icon used for the page on some platforms.
+        Return the icon to be used for the page on some platforms.
 
         Currently only used on macOS.
 
-        @note This method is only pure virtual on platforms that require it
-              (macOS). On other platforms, it has default implementation that
-              returns an invalid bitmap. The preprocessor symbol
+        @note This method is not pure virtual, but must be implemented on the
+              platforms that require it (macOS). The preprocessor symbol
               `wxHAS_PREF_EDITOR_ICONS` is defined if this method must be
               implemented.
+
+        @since 3.1.6
+     */
+    virtual wxBitmapBundle GetIcon() const = 0;
+
+    /**
+        @deprecated This function is deprecated, override GetIcon() instead.
      */
     virtual wxBitmap GetLargeIcon() const = 0;
 
@@ -187,7 +193,7 @@ public:
     Instead of reimplementing this behaviour yourself, you can inherit from
     wxStockPreferencesPage and get correct title and icon.
 
-    Notice that this class only implements GetName() and GetLargeIcon(), you
+    Notice that this class only implements GetName() and GetIcon(), you
     still have to provide the rest of wxPreferencesPage implementation.
 
     @library{wxcore}
@@ -215,5 +221,5 @@ public:
     /// Reimplemented to return suitable name for the page's kind.
     virtual wxString GetName() const;
     /// Reimplemented to return stock icon on macOS.
-    virtual wxBitmap GetLargeIcon() const;
+    virtual wxBitmapBundle GetIcon() const;
 };