Summary
-------

Add the methods *commitValue* and *cancelEdit* to the *DatePicker* class to provide methods which will either commit the value in the DatePicker TextField (set the text as value) or cancel it (set the text based off the value).

Problem
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In https://bugs.openjdk.java.net/browse/JDK-8150946, two new methods were added to ComboBox like controls with a TextField (*ComboBox*, *Spinner*): The *commitValue* and *cancelEdit* method. Those two method were simply forgotten for the *DatePicker*, so therefore there is currently no way to commit/cancel a value written in the *DatePicker* TextField.
This also caused a regression, which can be fixed via the proposed api.

Solution
--------

The proposed solution is to add the same two method to the *DatePicker* class as well (which is in fact also a ComboBox like control with a TextField).
Also a lot of new testcases will be added to test this solution.

Specification
-------------

The following two method will be added to the *DatePicker*. They are semantically the same as in *ComboBox* and *Spinner*

    /**
     * If the {@link DatePicker} is {@link #editableProperty() editable}, calling this method will attempt to
     * commit the current text and convert it to a {@link #valueProperty() value}.
     * @since 18
     */
    public final void commitValue() {
        ...
    }

    /**
     * If the {@link DatePicker} is {@link #editableProperty() editable}, calling this method will attempt to
     * replace the editor text with the last committed {@link #valueProperty() value}.
     * @since 18
     */
    public final void cancelEdit() {
       ...
    }