Add shortcuts

[floitschG]

I'm missing (or not finding) shortcuts to:

• switch from one editor-view to another.
• go to the next problem or search-result.

[stevemessick]

Florian always has two or three editors open side-by-side. A keyboard shortcut to cycle through the open, visible editors would help him a lot. Or two keys, one to move right, one to move left. A related issue is with navigation. Using hyperlinks is nice but it messes up his layout. He'd like to be able to open a definition in another visible editor stack, preserving his current visual context.

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Removed Type-Defect label.
Added Type-Enhancement label.

[pq]

Set owner to @pq.
Added Accepted label.

[clayberg]

Added Duplicate label.
Marked as being merged into #851.