[lmi] PATCH: Make code more clear by avoiding wxDCFontChanger

[Vadim Zeitlin]

 Hello again,

 As mentioned in a recent message in the "wx assertion failure; invalid
font" thread, here is another PR affecting more or less the same code area:

        https://github.com/vadz/lmi/pull/99

 This one is optional because it doesn't fix any user-visible problems, but
still worth applying IMHO because it gets rid of wxDCFontChanger which is a
fitting class when it's used for actually changing fonts, but not really
appropriate for the code here which only needs to preserve the current font
and not to change it.

 This change required moving the code previously using wxDCFontChanger from
ledger_pdf_generator_wx.cpp to pdf_writer_wx.cpp, but this is not a bad
thing neither, IMO, as like this all the code dealing with wxHtmlWinParser
directly is in the same source file now.

 Please let me know if you have any questions about or suggestions for the
further improvements of this PR.

 Thanks,
VZ