Copy and paste with clipboard
## Put data to clipboard
- [x] All selected text shapes are concatenated into string and added with `text/plain` MIME type
- [x] All selected shapes are encoded into json and added with `text/html` MIME type. (e.g. `<dgm>....data....</dgm>`

### Get data from clipboard
- [x] for `text/html` type, check `<dgm></dgm>` tag and convert to shapes
- [x] for `text/plain` and no `text/html` with `<dgm>` tag, insert into text shape
- [x] for `image/png`, `image/jpeg`, `image/svg+xml`, `image/webp` : insert into image shape
