Allowing to specify a template in the context of the new-tiddler event 

# If event.param is an object we look for a template property (this strategy is similar to where we look for a title property to use it as a title)
# If the template exists, we use it and its fields as template
# additional parameters may override or add fields

Try the following to see how it works

    title: new-tiddler test
    tags: displayonstartup

    Create a new tiddler the standard way as it has been before
    <$button>Click me!
    <$action-sendmessage $message="tm-new-tiddler" title="My new tiddler" tags="mytag [[another tag]]" testfield="123" />
    </$button>

    Create a new tiddler with a template and some fields that may override the template's fields
    <$button>Click me!
    <$action-sendmessage $message="tm-new-tiddler" template="$:/SiteTitle" title="Look at my body!" tags="mytag [[another tag]]" testfield="123" />
    </$button>


    Create a new tiddler with a template property that does not refer to a tiddler and just gets rendered as field
    <$button>Click me!
    <$action-sendmessage $message="tm-new-tiddler" template="blabla23" title="I have no template" tags="mytag [[another tag]]" testfield="123" />
    </$button>