change the default `actionName` to something that won't be ignored.

[tylercal]

In https://github.com/philc/vimium/commit/afeb65f5af849f04df8380212dbf50a6bf186bc4, a change was made to exclude elements with a jsaction with an actionName of "_". However, as written, this also excludes any jsaction that doesn't specify an action name. I'm not sure if this is a desired behavior.

As an example, on https://calendar.google.com/calendar/, each date in the mini calendar has a jsaction="click:nngp;" and each is clickable, but because no actionName is specified, Vimium does not place hints there.