Improve disabling of autocomplete and autofill

[Abestanis]

Improves our automatic ability to disable autocomplete/autofill.

There seem to be two systems which can display dropdowns on top of our dropdown:

• Autocomplete, which suggests words that were previously typed into a field.
• Autofill, which will suggest stored information (e.g. name, address, email...) for fields where it thinks it is appropriate.

autocomplete="off" will disable autocomplete, but not autofill. Any unrecognized value (e.g. autocomplete="chrome-off") will disable autofill, but not autocomplete.
There is a Stack Overflow answer which explains it a bit more in detail.

With this pull request we add some of Chrome' autofill heuristics and disable the autofill in these cases. With this patch I have not got a single suggestion dropdown menu from Chrome for the last three months.

[RoelVB]

Looks good. Thanks