Create Mirador plugin for item details button

[helrond]

Is your feature request related to a problem? Please describe.

The item details button is currently a separate component which is statically positioned on top of the Mirador header. This makes it hard to style in a responsive way and mesh with underlying Mirador styles.

Describe the solution you'd like

Add a Mirador plugin for this button so that the header can be styled as a whole.

Describe alternatives you've considered

It may be possible to add additional styles to Mirador without going the route of a plugin. Other solutions may also be available.