Add wxPersistentComboBox for saving and restoring combobox items

This allows to preserve the history of user input.

interface/wx/persist/combobox.h

@@ -0,0 +1,75 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/persist/combobox.h
+// Purpose:     Interface of wxPersistentComboBox
+// Author:      Vadim Zeitlin
+// Created:     2020-11-19
+// Copyright:   (c) 2020 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+/**
+    Persistence adapter for wxComboBox.
+
+    This adapter saves and restores the items of wxComboBox. A persistent
+    combobox can be used to preserve history of user entries.
+
+    Example of using it:
+    @code
+    // Suppose you need to ask the user to select their favourite Linux
+    // distribution, for some reason:
+    wxComboBox* combo = new wxComboBox(this, wxID_ANY);
+    if ( !wxPersistentRegisterAndRestore(combo, "distribution") )
+    {
+        // Seed it with some default contents.
+        combo->Append("Debian");
+        combo->Append("Fedora");
+        combo->Append("Ubuntu");
+    }
+
+    // Optionally, you might want to restore the last used entry:
+    combo->SetSelection(0);
+
+    @endcode
+ */
+class wxPersistentComboBox : public wxPersistentWindow<wxComboBox>
+{
+public:
+    /**
+        Constructor.
+
+        @param combobox
+            The associated combobox.
+     */
+    explicit wxPersistentComboBox(wxComboBox *combobox);
+
+    /**
+        Save the current items and value.
+
+        The current control value is saved as the first item, so that calling
+        @c SetSelection(0) when the control is created the next time will
+        restore the value which was last used. If the current value is the same
+        as one of the existing items, this item is moved to the front of the
+        list, instead of being added again.
+
+        If the current value is empty, it is not saved at all.
+
+        At most 10 items are saved, if the combobox has more than 10 items, or
+        exactly 10 items and the current value is different from all of them,
+        the items beyond the tenth one are discarded.
+     */
+    virtual void Save() const;
+
+    /**
+        Restore the combobox items.
+
+        This function doesn't change the current combobox value, you need to
+        call @c SetSelection(0) explicitly, after verifying that the combobox
+        is not empty using its IsListEmpty() method, if you want to restore the
+        last used value automatically. Otherwise the user can always do it by
+        opening the combobox and selecting it manually.
+     */
+    virtual bool Restore();
+};
+
+/// Overload allowing persistence adapter creation for wxComboBox objects.
+wxPersistentObject *wxCreatePersistentObject(wxComboBox *combobox);