Implement double/triple/quadruple click in TextArea

[darrenburns]

Note to self: Fix the behaviour when you click on whitespace.

Please review the following checklist.

• [x] Docstrings on all new or modified functions / classes
• [ ] Updated documentation
• [x] Updated CHANGELOG.md (where appropriate)

[willmcgugan]

@darrenburns Was this good to go?

[darrenburns]

I think I wasn't 100% satisfied with the behaviour without support for chaining on the mouse down event (there's a brief moment where the cursor jumps when going from a double to triple click, as it always moves on the mouse down event).

I think it may be good enough- but I recommend checking it out and having a play first.