Widgets: Idea: Add a screen-based joypad

Something similar to: https://bobboteck.github.io/joy/joy.html

But also with a slider bar to control zoom and focus:

It could even be used to control the ROV using two of then on the botton side with mobile devices.

The issue names no repository files or tests; start by reviewing the linked joypad example and the existing widget architecture. Clarify the interaction and platform requirements for the screen-based joypad, zoom and focus slider, and possible mobile ROV control before implementation. Done should include agreed behavior for those controls.

Written by the indexing model from the issue text.