#60 Widgets: basic_sheet_t needs to support enable()

[Foster Brereton]

The system isn't binding to cells that are in the
layout_sheet -- if it's not finding the cell it wants
in the Adam sheet, it'll fail altogether, even if the
cell exists in the layout sheet.

A sample of this is the first mini_image_size file, or
the optional_panel example.

[Foster Brereton]

Logged In: YES
user_id=1214256

The issue is that bind_controller_enabler presumes the cell
will exist in the Adam sheet, when this is not always the
case. The longer term fix is to make basic_sheet actually be
a model for the (yet to be fleshed out) property model
concept (e.g. being able to handle enablement aspects of
controllers).