Add support to load WGT packages on Tizen

This patch set will add support to load WGT packages on Tizen. It tries to do this by trying to change as little things as possible: basically, the WGT manifest is parsed and the same in-memory representation of a XPK manifest is created. From there on, things will work as they've been working previously.

Some other changes were necessary, as WGT packages contain their own application IDs (which are necessary for things like creating MessagePort channels), and they have a different format than what is generated during installation. Also, `tizen-extensions-crosswalk` are added to the command line if they're not there already.

This has been tested with an unmodified Bluetooth Chat program available from the Tizen repositories. It works as expected.

Some of this work has been made possible by following up on Riju's WGT work.