[Tizen] Dynamic Box installation support

This patch adds app-widget manifest handler.

BUG=XWALK-1515, XWALK-1516
Could I also ask you to change to StringToSizeType (since there are couple of other nits to be updated).
(I know I advised ToSizeType last time, but the semantics for it would be "TizenAppWidgetSizeType ToSizeType(const std::string& str_type)", if you want to keep the current semantics the name StringToSizeType is better. Sorry for not being clear)