Add keyboard shortcut for settings

[tzortz]

It will be extra useful for using firefox web apps.

[Jon]

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[towards_the_io]

If you set a bookmark on the settings page (`about:settings`) and add a keyword to it (e.g. `as`), you can open the settings page from the address bar with the keyboard sequence `<c-l>as<Enter>` (i.e. `ctrl+L` to focus the address bar, then type in your keyword `as` and then press `enter`).

Of course I am always in favor of more keyboard shortcuts and the ability to configure them, but maybe this will scratch the itch until then?