Plugin API: the tones of 0.8 for bars and dots too, and a green one

Plugin API 0.8 gives a plugin ui::set_tone() with ACCENT, BUSY (amber) and ALERT (red) for a tile's labels and icons. A plugin that shows a level (CO2, a battery, a price) also draws bars and dots, and needs the same colours for those, plus a green one for "good". Today a plugin can only reach them through theme::key_swatch() and theme::rgb(), which is not part of the plugin API.

A ui::tone_color(Tone) (or set_tone for a block's fill) and a Tone::GOOD from Home Assistant's green would cover it, in light and dark alike.

Stefan