Add common dialog wxCredentialEntryDialog

Prompting for user name and password is a common task.
At least windows provides these dialogs native but this
is just a generic implementation for now.

interface/wx/creddlg.h

@@ -0,0 +1,83 @@
+/////////////////////////////////////////////////////////////////////////////
+// Name:        creddlg.h
+// Created:     2018-10-23
+// Copyright:   (c) 2018 wxWidgets development team
+// Licence:     wxWindows licence
+/////////////////////////////////////////////////////////////////////////////
+
+/**
+    @class wxCredentialEntryDialog
+
+    This class represents a dialog that requests a user name and a password
+    from the user. It is implemented as a generic wxWidgets dialog on all
+    platforms.
+
+    @note For secure saving and loading, user and passwords have a look at
+        wxSecretStore.
+
+    @since 3.1.2
+
+    @library{wxcore}
+    @category{cmndlg}
+
+    @see @ref overview_cmndlg_cred
+*/
+class wxCredentialEntryDialog: public wxDialog
+{
+public:
+    /**
+        Default constructor.
+
+        Call Create() to really create the dialog later.
+    */
+    wxCredentialEntryDialog();
+
+    /**
+        Constructor.
+
+        Use ShowModal() to show the dialog.
+
+        See Create() method for parameter description.
+    */
+    wxCredentialEntryDialog(wxWindow* parent, const wxString& message,
+        const wxString& title,
+        const wxString& user = "",
+        const wxString& password = "");
+
+    /**
+        @param parent
+            Parent window.
+        @param message
+            Message to show on the dialog.
+        @param title
+            The title of the dialog.
+        @param user
+            The default user value.
+        @param password
+            The default password.
+    */
+    bool Create(wxWindow* parent, const wxString& message,
+        const wxString& title,
+        const wxString& user = "",
+        const wxString& password = "");
+
+    /**
+        Returns the entered user name.
+    */
+    wxString GetUser() const;
+
+    /**
+        Sets the current user name.
+    */
+    void SetUser(const wxString& user);
+
+    /**
+        Returns the entered password.
+    */
+    wxString GetPassword() const;
+
+    /**
+        Sets the current password.
+    */
+    void SetPassword(const wxString& password);
+};