Allow easy re-triggering of autocomplete to choose a different option

[microbit-mark]

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When you are using autocomplete, you get a useful dropdown to select pre-defined images for the Image class.

It would be useful to be able to select a new image from within your code, for example by clicking on the current Image to trigger the dropdown eg

[microbit-matt-hillsdon]

We also did some prototyping work that showed a drop-down arrow after the image, though good integration with autocomplete makes sense to me. One issue is that if you trigger autocomplete (Ctrl+Space) early in the word then it doesn't overwrite, so it would need to be a special mode related to autocomplete but not exactly the same.