Handled nested tags in strikethrough

Handled nested tags in strikethrough

The inner tag inherit the attributes of the outer tag and override them when defined in the inner tag
Example:
"<s height='5.0f' color='blue'> outer tag before  <s color='green'> inner tag </s> outer tag after </s>"
"outer tag before" and  "outer tag after" have height = 5.0f and color = 'blue'
"inner tag" has height = 5.0f and color = 'green'

This patch should be preceded by the patch below:
https://review.tizen.org/gerrit/c/platform/core/uifw/dali-toolkit/+/272264

Change-Id: I7426507b14c73f09aca67b9a22173e8ee5c0db29