extend edit widget to operate with subtiddlers

[buggyj]

With plugins being used as general containers for tiddlers there should be some way to edit them.

The visual editor plugin supports editing subtiddlers - the changes made to the code was simple: https://github.com/buggyj/tw5-tools/commit/5784677f4bac3ecf7f9a5fd4dec22b74a72d4606#diff-a8d94fd8e466a25ecefcf43f78f09e9c

[twMat]

For UI, the editor preview options could perhaps feature this.