[[ Bug 19764 ]] Ensure all canvas types have a describe method

[runrevmark]

This patch implements 'describe' methods for all canvas types meaning that they will no longer stop lists and other collections from printing out correctly if they contain a canvas type.

Note: The describe methods need more work in the future. However this patch solves the main issue - collections not describing if they contain a canvas type, so should be reviewed and merged as is.

[mention-bot]

@runrevmark, thanks for your PR! By analyzing the history of the files in this pull request, we identified @livecodefraser and @livecodeali to be potential reviewers.

[livecodeali]

Is there any reason this shouldn't go into develop-8.1? Also it needs a bugfix note. Also, it could have tests if this https://github.com/livecode/livecode/pull/5525 is merged, but that's probably optional :-)

[runrevmark]

@livecodeali : Sure - let's get #5525 merged in, then I'll try and backport and add tests.