qtconsole: Add vi-like key bindings to pager (fixes #6590)

Since _event_fileter_page_keypress() already exists in ConsoleWidget
class of qtconsole, that change was pretty easy and isolated. We emulate
up/down arrow keys with k/j when we use pager.

Suggestion. A few more vi/less key bindings can be added: h/l for
horizontal scroll, Ctrl-F/Ctrl-B for PageDown/PageUp. What do you think?