Reinstate Opera 12 as a supported browser

Everything is still broken, no RES code fixes
yet - those are in the following commits.
Opera 12: Fix refresh on load

Caused by setting window.location to itself.
Opera 12: Fix RESConsole JS error

Caused by having an element with the same ID as a global variable.
Opera 12: Fix loading resources

Use the opera.extension.getFile API on Opera 12.
Opera 12: Fix loading CSS

Caused by using a relative path within the extension package
instead of an absolute one.
Opera 12: Fix lightSwitch JS error

Caused by using the same name for a function and object property.
Opera 12: Fix constant refresh after loading

Caused by not removing the 'load' event handler after loading
"about:blank" in an iframe.
Consistently prevent default action of all handled hotkeys

This avoids conflicts with possible browser bindings
(e.g. if something is already bound to the J/K keys).