some TextboxHelper issues fixed

- add example for this issue: TextboxHelper.ClearTextButton does not
work when is applied via Trigger #1283
- remove inheritance to DependencyObject to avoid possible bugs with
attached properties
- change getter for HasText and add a setter
- fire HasText property on controls loaded event
- fix BindingExpression error for PasswordBox
- add example from @thoemmi for:  a search box with a magnifier on its
button, but as soon as text is entered, the button content should change
to an X.
Merge pull request #1371 from MahApps/punker76-fix-some-TextboxHelper-issues

some TextboxHelper issues fixed