added new submodule containing expected screenshot tests

.gitmodules

@@ -30,6 +30,9 @@
     path = plugins/QueuedTracking
     url = https://github.com/piwik/plugin-QueuedTracking.git
     branch = master
+[submodule "tests/UI/expected-ui-screenshots"]
+	path = tests/UI/expected-ui-screenshots
+	url = https://github.com/piwik/piwik-ui-tests.git
 
 # Note: when you add a submodule that SHOULD be left in the packaged release such as the few submodules below,
 #       then you MUST add these submodules names in the SUBMODULES_PACKAGED_WITH_CORE variable in:
@@ -39,4 +42,4 @@
 [submodule "libs/PiwikTracker"]
     path = libs/PiwikTracker
     url = https://github.com/piwik/piwik-php-tracker.git
-    branch = master
+    branch = master
\ No newline at end of file

expected-ui-screenshots @ ba39260e

+Subproject commit ba39260eec2c053e62c45c9bd2b327e84d54c566