make about dialog boxes use standard wx about dialogs

Any information in dialog can be easily changed without much effort
and the dialog can be easy translated.

Unfortunately on Windows there is no good standard about dialog,
therefore own implementation is used there.

                  <xsd:element name="Linux">
                    <xsd:complexType>
                      %(toolchain_gcc)s
                    </xsd:complexType>
                  </xsd:element>