Ignore fields which most likely aren't for credentials

There are some web fields which most likely the user will never want to
store in KWallet, such as search fields. However, currently the KWallet
integration also takes into account these, which can lead to surprising
results (for example, the wallet icon being displayed for pages like
Google which only has a search field).

To avoid this, each field name is checked aginst a set of hard-coded
field names which it's assumed the user will want to ignore.