Add page scroll up and down accel entries for terminal-widget in shortcuts editor (#221)
This MR is a solution to issue #221 (closed).
Previously there were no entries for handling the page up and down motion in the widget. The shift + page_up/page_down are vte defaults ig. So this MR simply adds two accels for handling the page up and page down. Now the user can set page up and down accels of their liking.