Improve the format of widget message parameters

In 5.1.3 we introduced the idea that the `event.param` variable could
optionally be a hashmap, allowing multiple values to be passed with the
message.

This change moves the hashmap to a separate `event.paramObject`
variable, allowing us to pass both a hashmap and a param string.
Update new tiddler message handling to accept title and fields

Now you can create a new tiddler with both a template and additional
specified fields.
Update modal handling to allow variables to be passed

@felixhayashi I’m afraid I haven’t used your pull request as there were
a few details that I wanted to do differently. The main change is that
these changes allow both a param string and hashmap to be specified.
Release note update