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.

MANIFEST.in

@@ -0,0 +1,9 @@
+recursive-include share *.png
+recursive-include share *.svg
+
+include flatcam.desktop
+include LICENSE
+include README.md
+include *.py
+include MANIFEST.in
+