opt-in for Mac integration

The Mac integration is not fully stable for now.

issue4595
review19341002

doc/usage.rst

@@ -49,6 +49,11 @@ Options
 
 -s SERVER, --server=SERVER           Specify the server hostname
 
+Environment
+***********
+
+`GTKOSXAPPLICATION`                  Activate with native Mac desktop
+
 URL
 ***
 

tryton/gui/main.py

@@ -42,9 +42,9 @@ import tryton.plugins
 from tryton.common.placeholder_entry import PlaceholderEntry
 import pango
 import time
-try:
+if os.environ.get('GTKOSXAPPLICATION'):
     import gtkosx_application
-except ImportError:
+else:
     gtkosx_application = None
 try:
     import gtkspell