Focus states and tweakpane accessibility improvement

This MR fixes accessibility issues with tweakpane and improves focus states across the app, including:

• All buttons and tweakpane controls have a visible focus state that is in contrast to the background. (#209 (closed), #203, #139 (closed))
• Colour picker options are not tabbable when closed (#209 (closed))
• Controls and buttons announce what they do (#210 (closed))

fixes #214 (closed) #211 (closed) #210 (closed) #209 (closed) #140 (closed) #139 (closed) #139 (closed)