Marker thresholds: Distances and Double Click don't always determine marking vs. panning

[saulgreenberg]

See MarkableCanvas.ImageOrCanvas_MouseUp. The issue is that

• a slow single click on almost the same spot won't show a marker
• a rapid 'throwing' pan may produce a marker.

The parameters have to be tweeked, or the algorithm altered.

[saulgreenberg]

Fixed in Release 2.2.0.3 by differentiating between single vs double clicks and panning drags