Fix doxygen error + Minor coverity issue resolve

Fix doxygen error + Minor coverity issue resolve

Due to the unknown issue, If we set SpanType::Value as enum class +
If we return const Value, doxygen compiled failed at Accessibility::Value.

Error message :

Configuration file '/home/leaf/work/dali-toolkit/build/tizen/docs/dali.doxy' updated.

/home/leaf/work/dali-env/opt/include/dali/devel-api/atspi-interfaces/value.h:88: warning: no matching file member found for
Value * Dali::Accessibility::Value::DownCast(Accessible *obj)
Possible candidates:
 'T DownCast(BaseHandle handle)' at line 275 of file /home/leaf/work/dali-env/opt/include/dali/public-api/object/base-handle.h
 'Application DownCast(Dali::RefObject *refObject)' at line 83 of file /home/leaf/work/dali-env/opt/include/dali/devel-api/adaptor-framework/application-devel.h

Since human cannot read this error, just avoid it.

+

Due to the doxygen make trouble since const value return,
I also try to remove all const return keyword that coverity issue will be occured..

TODO : Should FontSpan::GetFamilyName return const std::string& instead of const std::string?
That API is also kind of meanless const keyword btw.

See https://review.tizen.org/gerrit/#/c/platform/core/uifw/dali-toolkit/+/282555/ 's case 4.

Change-Id: I6e087e66a3de3ee4a5981bdc540616910ab04b6e
Signed-off-by: Eunki Hong <eunkiki.hong@samsung.com>