Numeric Key mode for quick key annotation

[erichs]

Toggling the 'numeric key' button auto-populates the label with a number, incrementing each time a label is placed. Hand-edit the number if you don't like the current value.

Also, set the initial label size to a relative percentage of the image width, for a better ballpark guess at a legible key label.

This makes it really easy to quickly key a map with numbers, if that's your preferred jam.

Here's a demo showing me keying a 5-room dungeon:
https://github.com/user-attachments/assets/af188f0e-3c96-48c0-af41-cb015b3a7d2d