Added New Methods in FormControls class and ErrorHandler class

ErrorHandler: Added InvalidControlException and
InvalidValidationException

FormControls: Added class PictureBoxes, TextBoxes, DataGridViews,
ComboBoxes, CheckBoxes and FormHandler

TestApp: Added TextBoxes examples and ComboBoxes examples
CheckBox "IsNull" Method name changed to "IsChecked"
Methods changed in CheckBoxes class, less methods but better overloads that make the same thing of the old methods in a better way.
Form checkboxes in TestApp were updated and is already running perfectly.
FormControls buttons' name changed
Changed messages from exception
Added comments
Removed datagridview class from test app