[Application] Parse WGT manifest as if it were a XPK manifest

Since the current code is too tied on an application being packaged in the
XPK format -- and thus assuming things are using a "low level" representation
of its metadata, it is simpler in this stage to just parse the XML file from
WGT widgets and create the same in-memory data that the XPK packages contain.
[Application] If loading from WGT, use provided Tizen App ID
[Application] Load application after it has been extracted