TextEditor: Remove one stack of EditorWidget constructors

There are conceptually only two: one that operates a new document,
and one that shares one. Being explicit makes moving data over to
the Editor hierarchy easier. Convenience can be re-added there,
later.

Change-Id: I9b34ff26628c99ffff01201dcf99332d5e7253e9
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>