ui: buttons to select-only or all-except each time series

[flyingmutant]

Shift-click is completely undiscoverable.

[leeoniya]

https://github.com/leeoniya/uPlot/blob/master/dist/uPlot.d.ts#L278

[flyingmutant]

https://github.com/leeoniya/uPlot/blob/master/dist/uPlot.d.ts#L278

Yep, we want to try to make it obvious for all users that we support both modes, because now only the power-users are aware about the second behaviour you get when holding Shift.