GtkApplication: Clarify docs a bit

Explicitly mention how gtk_application_set_accels_for_action can
be used to remove all accelerators for an action.

https://bugzilla.gnome.org/show_bug.cgi?id=729451
Small documentation fixes

Pointed out in
https://bugzilla.gnome.org/show_bug.cgi?id=729461
physically based kinetic scrolling
fixes
in the case of overshoot, check if the position is close to 0, instead of the velocity, to determine whether we are done
merged