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

This request is a result of new insight gained in the discussion of #1050.
The proposed changes go well along with #429.

Excerpt from #1050 

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It seems to me that many users really look for a feature to create a new tiddler with a chosen title via a message. Naturally they want to specify a template and add some variables.

Two plugins I know picked up this issue.

Stephan's plugin: https://groups.google.com/d/msg/tiddlywiki/EbTVXUrWss4/Wwg57Zmur2YJ
Matabele's plugin: https://groups.google.com/d/msg/tiddlywiki/sLYQCaTVRVA/hUqi9nx-c-YJ

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What is changed by this new pull request?

1) 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)
2) If the template exists, we use it and its fields as template
3) 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 discarded
    <$button>Click me!
    <$action-sendmessage $message="tm-new-tiddler" template="blabla23" title="I have no template" tags="mytag [[another tag]]" testfield="123" />
    </$button>