Release version 0.5.21

Release version 0.5.21

changes:
- Fix FindById bug
- Set current time when post notification
- Change the type of uid value int to uid_t
- Add SetAppLabel function for group item
- Fix bugs related to object creation
- Add Get/Set background info to Style
- change memory allocation for noti ex item when get item from manager handle
- Add module for notification ex item under the notification ex module
- Add send event api to send item list

Change-Id: Idc3da527d7d53164d7f09b93d637aba7015bd8c7
Signed-off-by: SukHyung, Kang <shine.kang@samsung.com>