Refactoring and documenting the markup tags and attributes

Moved all tags and attributes to separate file.
Added doxygen description for each tag and attribute.

To check doxygen documentation:
   1) Build dali-toolkit
   2)Generate the internal documentation for DALi Toolkit
     https://github.com/dalihub/dali-toolkit/tree/master/docs/generated-internal
   3) Goto: Files > File List > dali-toolkit > internal > text > markup-tags-and-attributes.h

Change-Id: I42693d0c1ce1169614ccad9391ef4f60a04b3f66