Add drop-support for include tags

Add drop-support for include tags

Add the include tag back into the palette, and add special drop
support for it such that when it is drop, a resource chooser pops up
and asks you which layout to include. A new validator prevents any
layouts from being chosen that would result in a cyclic
dependency.

This requires some infrastructure changes: First, drop handlers must
distinguish between a view getting created as part of a previewing
operation and getting created interactively. Second, in order to
support cancel removing an inserted include if the user decides not to
set an include, the node wrappers need to support removing an element.
Also, use the metadata originally intended for the preview icon
factory to also bypass palette drag previews for widgets that don't
have UI.

Change-Id: I1bdd1766ca4cfa2fdbca25b77c50c74e9c332cbd