Control initial variable in timeseries widget

[visr]

The first variable (the first numerical column) is automatically selected. In Ribasim that was storage, but most people want to look at level, so I wanted to have that selected by default to save users a few clicks.

I worked around this by making level the first numerical column, but perhaps having a way to control this would be good as well.
See Deltares/Ribasim#1971.