[FEATURE] Legend.Item Transparency on Disabled

## Current Behavior
When a user clicks on a legend item currently a strikethrough effect is placed through the text of that legend item and the coloured box of the legend item remains unchanged.

## Possible Solution
I have done a quick proof-of-concept (which could easily be incorporated into the main repo) which changes the alpha channel on the existing box and text colours to increase the transparency.

The questions are:
- Is this feature considered useful by the community?
- If so, should there be additional configuration options exposed (preferred) so a user can provide `dataset.disabledBackgroundColor`, `dataset.disabledBorderColor` and `legend.labels.disabledFontColor` at the expense of adding additional configuration options or should it be guessed?