[WIP] [Application] Expose installed applications via D-Bus

Adds InstalledApplicationsRoot which represents the '/installed' object
path in 'org.crosswalkproject' service, that contains child
objects (both in D-Bus and C++ "trees") representing installed
applications.

Those child objects are implemented using InstalledApplicationObject.

TODO(cmarcelo) Add the D-Bus interface description in the comments and a
quick summary here.