[Tizen] Add unit test for app-widget configuration element handler

This patch adds tizen_appwidget_handler_unittest.cc. New tests verify
that the handler properly handles the following configuration elements:

widget.app-widget @id @primary @update-period @auto-launch
widget.app-widget.box-label @lang #text
widget.app-widget.box-icon @src
widget.app-widget.box-content @src @mouse-event @touch-effect
widget.app-widget.box-content.box-size @preview @use-decoration #text
widget.app-widget.box-content.pd @src @width @height

BUG=XWALK-1515, XWALK-1516
Introducing this function shortens the code and makes it easier to read. Otherwise whenever this function is used at the moment (at least 17 places) I would have to put
if (!value.empty()) dict->SetString(key, value);
I need element creation functions that in some cases do not set a value of a key at all.
Another thing is test for empty key and null dictionary. Those should be put on DCHECK, but I dont think it will work in this kind of tests. Am I right?