[taggle] Brushing continues even after mouseup outside of the vis

[thinkh]

• Release number or git hash: mothertable branch

Steps to reproduce

1. Start brushing a few items (barplot, list or heatmap1d)
2. Release the mouse button outside of the visualization/multiform
3. Hover over the items again

Observed behavior

• Items will still be selected, eventhough the mouse up was done outside

Expected behavior

• The brushing should stop when the user clicks outside