add python setuptools support

- add setup.py script,
 - switch to use images via "share:" resource name
   (this allows to work both versions of application:
     - system wide installation,
     - directly from source),
 - add .desktop file,
 - rename FlatCAM.py to flatcam and install it as an
   executable script.

flatcam.desktop

@@ -0,0 +1,8 @@
+[Desktop Entry]
+Name=FlatCAM
+Comment=2D Computer-Aided PCB Manufacturing
+Exec=flatcam
+Icon=flatcam
+Keywords=cam;pcb;gerber
+Terminal=false
+Type=Application