[FEATURE] Add an option to smooth the pressure in Weighted Smoothing mode

This feature makes the shape of the line be more stable, but still there
is one drawback:

When the option is active you almost cannot control the shape of the
beginning of the line, it is always very thin.

CCMAIL:kimageshop@kde.org

Rename smooth parameter

Renamed "Tail Aggressiveness" option "Stroke Ending"

Other nice variants we considered:

-"Tail Aggressiveness"
-"Tail Chasing"
-"Goofiness"

:)

Renamed "Smooth Distance" to "Distance"