Add Xuanxuan IM app. (electron#241)

* Add Xuanxuan IM app.

* Fix errors in yml file.

* Remove screenshots field.

Author: catouse

apps/xuanxuan/xuanxuan-icon.png

@@ -0,0 +1,12 @@
+name: 'Xuanxuan IM'
+description: 'An open source IM resolution.'
+website: 'http://xuan.im/'
+repository: 'https://github.com/easysoft/xuanxuan'
+category: Productivity
+keywords: 
+  - im
+  - messaging
+  - collaboration
+  - office
+locales:
+  - zh-CN