[Runtime] Separate WGT application manifest handling from XPK applications.

When loading WGT manifest file, current Crosswalk maps XML Widget
elements to manifest keys by hard code. Consequently, WGT app and XPK
app share the same manifest keys and manifest handlers. As with more
manifest keys and handlers are introduced for WGT app, this causes a
mix up of WGT manifest keys and XPK manifest keys, due to WGT and XPK
app have different manifest formats and keys, especially put Tizen
Extending Configuration Elements into consideration.

A solution to fix above issue is to separate WGT manifest keys and handlers
from XPK apps. In detail, when loading WGT manifest, we introduce a
mechanism to save XML configuration information into manifest Dictionary
structure(note that this is not hard code mapping), keys are different
from those of XPK apps. Then we could add WGT specific handlers for
these keys.