Gtk UI: fixed lookup of icons and fallback.css on Windows

* Since we don't have an absolute and known installation directory,
  we should look for these files in the same directory as sciteco.exe.
* For the time being, we look for them in $SCITECOCONFIG which defaults
  to that directory.
* Cannot be a final solution as you may tweak $SCITECOCONFIG to
  fit an Unix-like environment (eg. set SCITECOCONFIG=$HOME).
  In such configurations it may also not be suitable to always
  look in the directory of sciteco.exe since that may be some /bin dir.
  Considering that GTK+ forces us to preserve some kind of UNIX-like
  directory hierarchy even for portable builds, we should perhaps
  install the icons into the hicolor icon theme.
  This would also simplify Debian packaging.

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