Rename extension instance 'wxHMI' to 'wxglade_hmi'

It is necessary because of following commit 7f59aa398669 ('WxGlade HMI extension now does
only instanciate wx object who's name match extension's name. If no
object match the name and no code is provided in 'start' user python
code section, then a warning is issued')

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                  <xsd:element name="Win32">

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                    <xsd:complexType>

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                      %(toolchain_gcc)s

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                    </xsd:complexType>

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                  </xsd:element>

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