Add mnemonics to the tabs

This improves keynav, and doesn't hurt.
Properly present the window

This doesn't make a big difference, but it is the proper
way to show a window.
Add a utility function for focus grabbing

Grabbing focus to a widget is a little tricky, since it
must be mapped for that to work. Add a utility function
that waits for that to happen.
Grab the focus to the scrolled windows on the page

This makes page up/down work when you get to the page.
Also, scroll the overview back to the top every time.