GTK: Numeric icons to indicate tab position

[mitchellh]

Discussed in #2624

^{Originally posted by jcollie November 8, 2024}
As discussed on Discord: https://discord.com/channels/1005603569187160125/1304093836439457842

The proposal is to add icons to the 1st 10 tab headers to indicate the tab position, making it easier to switch tabs using keybindings (alt+one..alt+zero). This is what it would look like:

Initially I was using icons taken from https://pictogrammers.com/ but I liked ones that I drew better, but I'm sure that a more experienced designer could improve on my efforts.

This would be gated with a configuration entry, which would default to not showing any icons.