Smoothening threshold in new canvas should be customizable

[DarkDaskin]

Currently the smoothening threshold is hardcoded to 1 which produces terrible results when drawing with a mouse slowly. For me the value of 3 produces best results, but it may depend on input manipulator used and computer performance, so it should be customizable.

Ideally the new canvas should have hotkeys to tune the value dynamically, while the initial value is set on options page.

[grompe]

It's a bit of misnomer, because it's point reduction treshold, after which lines appear smooth due to another function (buildSmoothPath).

While it's worth tuning the best parameters for algorithms there are now, need to keep in mind two constraints: bloating the recording with too many points, and making output too different from the old canvas.

For best algorithms, should look into how real painting programs, such as Krita, do it: https://bugs.kde.org/show_bug.cgi?id=281267

That said, adding an option to set the threshold from 1 to 4 should be good enough for now.