Button type form element doesn't have #value property

Problem/Motivation

When creating a button form component, the Element title is set to the #title property, but in the base of Button type elements, it should probably be set to the #value property.

Steps to reproduce

Create a button, give it an "Element title", view the form, and note that the button has no text.

Proposed resolution

When it is a Button type element set the #value property.

Remaining tasks

The MR.

User interface changes

Button text!