settings: add preferences for scroll on output/keystroke

These are commonly tweaked and so it makes sense to expose them in the preferences along other needs like audible bell.

While doing this, I notice that scroll-on-output in VTE alone does not work when used with a GtkScrolledWindow while kinetic scrolling is active. Nothing is causing the GtkAdjustment animation code to stop. That may be something we want to address separately since that comes about from the VteTerminal/GtkScrolledWindow combination.