Add mutex for set repeat text

Add mutex for set repeat text

- Issue:
Sometimes, invalid access occurs if app tries to add text in multi
thread enviroment.

- Solution:
App can add text frequently. And also, app can add text in multi thread
enviroment. However, the text_repeat property of client is not safe in
multi thread enviroment. Thus, this patch adds mutex lock for protect
text_repeat from race condition. Through this patch, app can be add text
safely in multi thread.

Change-Id: Iffcce54fcbc34436bf9f4d7ec7aadf571f05889b
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>