Add "astree.png" (app icon), "astree.desktop" and "astree.appdata.xml" files

For example, `astree.desktop`:

```
[Desktop Entry]
Type=Application
Name=Astree
Exec=astree %f
Icon=astree
Terminal=false
Categories=Graphics;Engineering;Education;
MimeType=application/x-astree;
StartupNotify=true
```

For example, `astree.appdata.xml` (created using @probonopd's [generator](http://output.jsbin.com/qoqukof))

```

astree
CC0-1.0
LGPL-3.0
Astree
Free open-source optical ray tracing and design software

Astree software computes and shows the ray-tracing analysis of optical devices, having conical or polynomial surfaces,having reflections, transmissions, stops, analysis of the image quality.

astree.desktop
http://edeforas.free.fr/


http://edeforas.free.fr/Astree/astree.png



astree.desktop

```

The requested files are astree.png, astree.desktop, and astree.appdata.xml. Start by reviewing the examples in the issue and the repository's existing installation or packaging layout to determine where they belong. Done means all three files are included with metadata identifying Astree and its desktop launch details.

Written by the indexing model from the issue text.