Add wxDD_MULTIPLE wxDirDialog style and implement it for GTK

Add wxDirDialog::GetPaths() similar to the existing member of
wxFileDialog with the same name and also taking, for consistency,
wxArrayString as the output parameter.

interface/wx/dirdlg.h

@@ -7,6 +7,8 @@
 
 #define wxDD_CHANGE_DIR         0x0100
 #define wxDD_DIR_MUST_EXIST     0x0200
+#define wxDD_MULTIPLE           0x0400
+
 #define wxDD_NEW_DIR_BUTTON     0 // deprecated, on by default now,
 
 #define wxDD_DEFAULT_STYLE      (wxDEFAULT_DIALOG_STYLE|wxRESIZE_BORDER)
@@ -43,6 +45,11 @@ const char wxDirDialogNameStr[] = "wxDirCtrl";
     @style{wxDD_CHANGE_DIR}
            Change the current working directory to the directory chosen by the
            user.
+          @note This flag cannot be used with the @c wxDD_MULTIPLE style.
+    @style{wxDD_MULTIPLE}
+           Allow the user to select multiple directories.
+           This flag is only available since wxWidgets 3.1.4
+
     @endStyleTable
 
     Notice that @c wxRESIZE_BORDER has special side effect under Windows
@@ -109,6 +116,16 @@ public:
     */
     virtual wxString GetPath() const;
 
+    /**
+        Fills the array @a paths with the full paths of the chosen directories.
+
+        @note This function should only be used with the dialogs which have @c wxDD_MULTIPLE style,
+              use GetPath() for the others.
+
+        @since 3.1.4
+    */
+    virtual void GetPaths(wxArrayString& paths) const;
+
     /**
         Sets the message that will be displayed on the dialog.
     */