more reliable cut/paste detection for editor undo/redo

[liucougar]

listen to oncut/onpaste events in Editor (IE and safari already support these while FF 3 will support them as well), so we do not need to listen to keyboard shortcuts

we need to remove undo/redo detection code for paste/cut in execCommand function when invoking paste/cut by clicking toolbar buttons

(for FF2, there is no way to detect cut/paste if using context menu. the only way around that is to provide custom context menu)

[liucougar]

fixed in r13273 (wrong ticket no. in that log)