Allow alpha in color annotation picking

[lpatiny]

Currently we can not decide the transparency of the ROI annotations and this would help the visibility.

Could you add this feature and set it by default to 0.5 ?

Also by default we should have as text-size: 9px and only the 'surface' should be visible for the text annotation.

[lpatiny]

The alpha control is in a storybook on react-science:

https://react-science.pages.dev/stories/?path=/story/components-color-pickers--color-picker-story

Please check

https://stackoverflow.com/questions/6042550/svg-fill-color-transparency-alpha

I prefer you don't use the RGBA format in the SVG because it is not supported (yet) by all the softwares like inkscape.