Support manipulating window tabs

[tmandry]

The AX APIs make it pretty easy to manipulate tabs in windows. I can see this being useful in x3 e.g. for setting a "mark" on a browser tab, rather than just the browser window itself.

This might belong in another library as sort of an extension to Swindler. I'm not sure if there's any app-specific stuff that goes into it, but it does feel sort of outside the core functionality of Swindler that I would want to maintain.

For reference see Hammerspoon implementation.