config: add bar.trackpad-sensitivity

This is an integer that specifies the amount of scrolling that needs
to be accumulated before a wheel-up/down event is emitted.

A higher value means you need to drag your fingers a longer distance
before the event is emitted.

The default is 30.

bar/private.h

@@ -10,6 +10,7 @@ struct private {
     int height;
     int left_spacing, right_spacing;
     int left_margin, right_margin;
+    int trackpad_sensitivity;
 
     pixman_color_t background;