NewDialog: Simplfy code for templates view's signals

We don't need to connect to the clicked signal, since we are already
connected to the current changed of the selection model. Also set the
model for the templates view in the constructor, so that we can also
connect to the selection model in the constructor.

Change-Id: I88cd1bef97136604125fc74e18ffcc5c1c118a1d
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>