feat: Make label text editable

When the cursor hovers over text, a text selection cursor shows up (this is the default browser behavior, so that the text can be selected).

It would be great if, in addition to selecting/copying, you could actually click and edit the label text.

It's clear how these edits would be propagated back to the Substance program: just modify the label string.

What's less clear (as usual) is how these edits would impact the optimizer state. E.g., should the UI wait for the edit to finish (e.g., wait for the return key) and then re-optimize? Use the current state as an "anchor" and find the minimal adjustments that account for the new size of the text? Etc.