Initial check-in for the new release branch for UPM support

Author: SimonDarksideJ

package.json

@@ -8,7 +8,7 @@
   "name": "SimonDarksideJ",
   "twitter": "@SimonDarksideJ"
 }],
-  "unity": "2019.1",
+  "unity": "2019.3",
   "repository": {
   "type": "git",
   "url": "git+https://bitbucket.org/UnityUIExtensions/unity-ui-extensions.git"