Change legendTemplates to use separate spans for text and icon.

Change legendTemplates to use separate spans for text and icon.

This addresses the issue in #2192 without entirely reverting #886. Note that it still may cause confusing behavior for some legends: Specifically, the example on chartjs.org styles ".doughnut-legend li span" with a fixed width, and therefore both of the new spans will be impacted by this styling. However, now two spans are generated instead. %name%-legend-icon is used for the color icon, and %name%-legend-text can be used for styling the text. This should be described in release notes.