Add support for custom comboboxes in wxFileDialogCustomize

Allow using simple (i.e. not editable) comboboxes, known as wxChoice in
wx API, in the dialog too.

Demonstrate their use in the dialogs sample.

interface/wx/filedlgcustomize.h

@@ -109,6 +109,32 @@ public:
     void SetValue(bool value);
 };
 
+/**
+    Represents a custom read-only combobox inside wxFileDialog.
+
+    Objects of this class can only be created by
+    wxFileDialogCustomize::AddChoice().
+
+    It is possible to bind to wxEVT_CHOICE events on this object, which
+    will be generated when the selection in the combobox changes.
+
+    See wxFileDialogCustomControl for more information.
+
+    @since 3.1.7
+ */
+class wxFileDialogChoice : public wxFileDialogCustomControl
+{
+public:
+    /// Return the index of the selected item, possibly wxNOT_FOUND.
+    int GetSelection() const;
+
+    /// Set the selection to the item with the given index.
+    ///
+    /// Using @c wxNOT_FOUND for @a n is not supported, once a selection is
+    /// made it cannot be undone.
+    void SetSelection(int n);
+};
+
 /**
     Represents a custom text control inside wxFileDialog.
 
@@ -203,6 +229,21 @@ public:
      */
     wxFileDialogRadioButton* AddRadioButton(const wxString& label);
 
+    /**
+        Add a read-only combobox with the specified contents.
+
+        The combobox doesn't have any initial selection, i.e.
+        wxFileDialogChoice::GetSelection() returns @c wxNOT_FOUND, if some item
+        must be selected, use wxFileDialogChoice::SetSelection() explicitly to
+        do it.
+
+        @param n The number of strings, must be positive, as there is no way to
+            add more strings later and creating an empty combobox is not very
+            useful.
+        @param strings A non-@NULL pointer to an array of @a n strings.
+     */
+    wxFileDialogChoice* AddChoice(size_t n, const wxString* strings);
+
     /**
         Add a text control with an optional label preceding it.