Improve the underline markup

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

2) Enhanced the performance of underline-runs and reduced the needed memory to store glyphs-runs

This patch should be preceded by the patch below:
https://review.tizen.org/gerrit/c/platform/core/uifw/dali-toolkit/+/271685

Change-Id: I529aacd66c83fb1e55e282a7b849c901c3e0c67a