ADT: support for editing embedded layouts.

ADT: support for editing embedded layouts.

This is a very first rough changeset. The context in which the layout
is edited is hardcoded until we have the UI for it.

The basics are the following:
- if there's a context, don't pass the parser on top of the model to
  the layout lib.
- instead create a KXml based parser that will return the model-based
parser when queried for a custom parser (if the layout name matches the
file being edited).

Change-Id: I2d106def9c64e82ab50c8fae96cb050338422a85