Add Gimp sources for xgterm and ximtool icons

The sources are 512x512. They can be converted and scaled with
ImageMagick:

convert XGTerm.xcf \
  -background transparent -flatten \
  -scale 128x128 \
  xgterm.png

Linux (freedesktop) wants to have icons in 16x16...512x512 as optimum.

For macOS, one needs to include a transparent border around it:

convert XGTerm.xcf \
  -background transparent -flatten \
  -bordercolor transparent -border 5% \
  -scale 128x128 \
  xgterm.png