[D-Bus] Add ObjectManagerAdaptor

A simple implementation of the "server" side of
org.freedesktop.DBus.ObjectManager interface. ObjectManagerAdaptor
manages the object subtree and provide add/remove managed objects, that
inherit from ManagedObject (if needed).

Extracted from InstalledApplicationsManager and we will be used to
implement RunningApplicationsManager.
[Application][Linux] Use ObjectManagerAdaptor for '/installed' interface

ObjectManagerAdaptor was created based on InstalledApplicationsManager,
so we now can use it to clean up things a bit. The

One non-trivial change is that the object path now is being defined by
the Manager instead of the individual objects. This was needed so we can
put the finding code in the ObjectManagerAdaptor.
Merge pull request #1271 from cmarcelo/extract-object-manager

[D-Bus] Create ObjectManagerAdaptor