don't use flatpak env variable

flatpak/uberwriter.json

@@ -18,8 +18,7 @@
     ],
     "build-options" : {
       "env": {
-        "PYTHON": "python3",
-        "IN_FLATPAK": "1"
+        "PYTHON": "python3"
         }
     },
     "add-extensions": {

setup.py

@@ -36,7 +36,7 @@ from pprint import pprint
 pprint(extra_files_ui)
 pprint(extra_files_media)
 
-if os.environ.get('IN_FLATPAK'):
+if os.path.isfile("/.flatpak-info"):
     app_prefix = '/app/'
 else:
     app_prefix = ''

uberwriter_lib/uberwriterconfig.py

@@ -55,7 +55,8 @@ def get_data_path():
     """
 
     # Get pathname absolute or relative.
-    if os.environ.get("IN_FLATPAK"):
+    # TODO: Abstract this (the old env IN_FLATPAK)
+    if os.path.isfile("/.flatpak-info"):
         return '/app/opt/uberwriter/data/'
 
     path = os.path.join(