fix: ensure that each mirador instance has its own namespace for classnames

[fbessou]

Mirador defines its styles as json and delegate classnames generation to the @material-ui/core/styles library. In production, the generated class names are deterministic and unique into a given StylesContext. Each mirador instance creates a new StylesContext, leading to classnames not being unique accross several Mirador instances. Mirador exposes the MUI's createGenerateClassNameOptions which allows tweaking the generation of class names. Here we add a unique prefix to every Mirador instances through the use of a counter.

[vicpsl]

Integrated in commit 1c5500912135799ab25ca63fd026f8f37f6c3dfb. Closes #131.