Improved UX for editing labels in GUI

[tturocy]

This adds some improved UX in the GUI for inputting labels:

• Leading whitespace and multiple whitespace are normalised on the fly while editing
• Trailing whitespace is normalised transparently on commit
• For now, non-ASCII input is filtered on the fly; this has been written to make it easy to switch to Unicode for 17.0 as planned but aligns with the current rules.

Closes #945.

[tturocy]

@rahulsavani @d-kad If you can try out the various places we set/edit labels that would be helpful.

[d-kad]

Tested on macOS

Action labels work fine (you can see that action label turned "Éxït" into "xt" and removed trailing spaces from the second label automatically)

Node labels through on invalid input

[tturocy]

I wasn't able to reproduce this with the node labels; I did change that editor and on mine I'm not able to input invalid labels. Did you build yourself from the branch? If so are you sure you've got the latest version of it?

[d-kad]

You're right — I was three commits behind on issue_945, everything works as expected now.

Sorry for raising a false alarm!