Remove double "webform" from token names

Tokens currently have the rather awkward format [webform:webform-*]. I want to make a backwards-compatible conversion to better token names, like [webform:*].

1. It should be that webform_tokens_tokens() only advertises the new format token names, but both old and new are replaced by webform_tokens_tokens(). This allows current installations to work but encourages the use of the new tokens. My current thinking is to support both formats for D7 and only support the new format in D8.

2. The meta tokens should probably be broken off into their own hierarchical group.