[BrowserWindow] Enable single key shortcuts

Translate single keys to shortcuts:

1 - previous tab
2 - next tab
(basic Opera compatibility)

There are more to choose from, but 1 and 2 are probably the most used.
Full list:
http://help.opera.com/Windows/9.50/en/keyboard.html#single-key

Configurable in preferences/Keyboard shortcuts, off by default.

Closes #1172

Note: I don't know how to add a help text/tooltip to the checkbox, 'single key shortcuts' would be very terse on what's implemented, so the current two are mentioned.