Add label gun gui behaviour for clearning all of one specific label

[TeamDman]

https://discord.com/channels/967118679370264627/1436840493739937822/1436844118419443725

Ethon asks:

is there any way for me to just clear a particular label of all the associated blocks or do I just have to clear all my labels?

Could add a shift-click in the label gun gui with a confirm dialog to purge specific labels perhaps