Fixes gui-cs#3189. TextValidateField doesn't show the cursor and does…

…n't have TextChanged event. (gui-cs#3190)

* Prefix private fields with underscore and code cleanup.

* Rename to SetInitialProperties and fix a typo.

* Cleaning code and some adjustments.

* Fix default width for view.

* Enable cursor visibility.

* Reduces indentation and remove AutoInitShutdown.

* Add unit test for default Width is always equal to the provider DisplayText.Length.

* Also prevents showing the cursor if position is less than 0.

* Add OnTextChanged method and TextChanged event.

* Remove unnecessary TextChanged.