Fixes #6 canvas editing GUI didn't work for Igor and later

[msakuta]

Fixes #6, SketchCanvas editor GUI stopped working since Igor, due to some changes in the way forms are handled.
Specifically, these changes were incompatible:

• HTML_EDIT_FORMSELECTION event is not invoked anymore.
• New EDIT_FORM_ADDTEXTAREA event uses a new Form class instead of Doku_Form, which completely changes API.

Previously, we injected a canvas and its accopanying small JavaScript code to synchronize the text area and the canvas. However it is no longer possible to directly inject text into the output HTML. We need a specific implementation of Element class.

In order to keep backwards compatibility, HTML_EDIT_FORMSELECTION was left intact, and added a new EDIT_FORM_ADDTEXTAREA handler with the custom CanvasElement class.

Tested on:

• Igor
• “Jack Jackrum”
• “Hogfather”
• “Greebo”