Document keyboard shortcuts

[cweiske]

There should be a keyboard shortcut to focus the documentation search text field.

[cweiske]

Already implemented with #25 - Ctrl+K does it.

It would be nice to have that

1. documented on http://zealdocs.org/usage.html
2. a menu entry so I can discover it in the app itself.

[trollixx]

Ctrl+L is also supported now, still need to update the docs.

[trollixx]

Repurposing this for documenting all shortcuts.

[dmocek]

I've started documenting the shortcuts here: https://github.com/dmocek/zeal-keyboard-shortcuts.

The ones listed are all I have (and some, like Zoom, don't work for me).

[dmocek]

I've completed the list of shortcuts. These are all I know at this point: https://github.com/dmocek/zeal-keyboard-shortcuts.