Changes made to allow for harmonization of vcl/inc/impfont.hxx and
vcl/inc/fontattributes:

- comment added showing the similarities between impfont.hxx and
  fontattributes.hxx (made in fontattributes.hxx) - extra line
  space added for my own convinience as I tend to switch between
  tabs listing vcl/inc/impfont.hxx and vcl/inc/fontattributes.hxx
  in OpenGrok because that's how I roll

- fontattributes.hxx doesn't need rtl/textenc.hxx, that's already
  included in rtl/ustring.hxx, so swapped to use this instead

- fontattibutes.hxx now includes i18nlangtag/languagetag.hxx - this
  will be needed when I add the LanguageTag properties that exist in
  impfont.hxx

- unotools/fontdefs.hxx added to:
  + vcl/source/font/PhysicalFontCollection.cxx
  + vcl/source/font/PhysicalFontFamily.cxx
  + vcl/source/font/PhysicalFontFace.cxx
  + vcl/source/font/font.cxx

  These are the only specific files that actually need this include.
  Thus this has been removed as an included header from
  fontattributes.hxx

- vcl/inc/fontinstance.hxx doesn't need the reference counter to be
  a sal_uLong, a sal_uInt32 will suffice (if we need more references
  than that then I suggest there is a problem somewhere in the code!)

- rearranged vcl/source/font/font.cxx's included headers

Change-Id: I8d61a5c05989356c056b962adc5aa670940addd2