Slideshow Block: add new option to choose how navigation should appear (dots, numbers, thumbnails)

#### Is your feature request related to a problem? Please describe.
I'd prefer to see thumbnails instead of dots, as they're better for re-establishing context of what I've already seen and want to go back to.

#### Describe the solution you'd like
An option within the block's settings

#### Describe alternatives you've considered
A different plugin

#### Additional context

Start by locating the Slideshow block and its block settings, then inspect how its current dot navigation is rendered. The work is complete when the settings offer dots, numbers, and thumbnails and the slideshow navigation reflects the selected option.

Written by the indexing model from the issue text.