With a path selected that the Smooth tool can change, Alt held at a
Paintbrush press now switches the drag to the Smooth tool: it smooths the
selected paths under the drag with the Smooth tool's radius and Smooth Tool
Options' Fidelity, and the cursor shows the Smooth tool before the press.
Alt pressed after a stroke begins still closes the path on release. With no
such path selected (nothing, only text or an image, or only a two-point
line), Alt at the press draws the closed stroke as before.
The Paintbrush no longer reads the Pencil's "Alt key toggles to Smooth Tool"
setting: its Alt close and Alt smoothing do not depend on it.