feat(ui5-shellbar): introduce accessibilityTexts property

[ilhan007]

New property from type Object, called accessibilityTexts is introduced to allow customization of some internal parts of the ShellBar web component via stable API. Currently the only thing available on the object is the profileButtonTitle to define the profile button tooltip, showing up on hover.

Fixes: #4009

In a plain html, using the property would look like this:

<script>
    sbAcc.accessibilityTexts = {
	  profileButtonTitle: "John Dow",
    };
</script>

But in a React or similar programming model environment, one should be able to bind a object to this property in the template in a declarative manner.