Fix speed smoothing algorithm not to generate blobs during the stroke

There were the following problems:

1) KisPaintingInformationBuilder::hover() geneated distance noise
   while the user was painting on screen. Now it doesn't add sample
   points when the stroke is started, only while hovering.

2) KisSpeedSmoother now has longer buffer. 10 samples was too low value
   for tablets emitting events every 7 ms. 512 should be enough for
   everyone!

3) Now there is a minimal smoothing time range. It is set to 15 ms.

BUG:363364,375360