Add: message source

It would be nice to be able to see the source of a message. Perhaps hovering over a message could display the source instead, and clicking could populates the text box with that source.

For example, if i enter some complicated expression, and you want to correct a mistake, or change it slightly, it would be useful to be able to click my message and populate your text box with the TeX source of mine, so you don't have to re-type all the expression.

This would be particularly nice with lots of nested markup (fractions, sums, etc).