[Runtime] Generate the WGT package id by settings in config.xml.

For WGT package, the package id in Crosswalk runtime should be generated
from the settings (id attribute in widget (W3C widget) or
application (Tizen widget) element) in config.xml, instead of generating
from a randomly temporary directory path, so that one package should be
identified by its id.
Merge pull request #1628 from takethathe/wgt_id

[Runtime] Generate the WGT package id by settings in config.xml.