Give TextEditorActionHandler an explicit parent and unify usage.

The action handler implicitly passed ownership to ICore, which is non-
to the action handler. We now consistently create the action handler in
the editor factory, give ownership to the editor factory, and don't hold
a reference to it.

Change-Id: I4372f8de966e3ceff87c06c5528c6b54522c1d57
Reviewed-by: David Schulz <david.schulz@digia.com>