Add wxCheckListBox::GetCheckedItems() helper.

This method is similar to wxListBox::GetSelections() and allows to retrieve
all checked items at once.

Closes #14969.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73422 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/checklst.h

@@ -37,6 +37,8 @@ public:
     virtual bool IsChecked(unsigned int item) const = 0;
     virtual void Check(unsigned int item, bool check = true) = 0;
 
+    virtual unsigned int GetCheckedItems(wxArrayInt& checkedItems) const;
+
     wxDECLARE_NO_COPY_CLASS(wxCheckListBoxBase);
 };