WindowSigils: Fixes and support for stacking windows and empty spaces

[eraserhd]

This incorporates the commit from #239 (thanks @erbridge) , and #239 should partially fix #238.

Window Stacking

In addition, I had some nice improvements that I never submitted that allow stacking windows nicely. When two windows have the same position:

1. Use the OS window id when sorting to keep the assigned sigils stable. (It used to depend on tab order, and so they would keep moving around).
2. Prevent the sigil indicators themselves from overlapping on the screen. They are now spaced out nicely so you can select a window as though it were a kind of tabbed control.

For example:

A Note: Planned Future Features

Not a part of this PR, but coming up in hopefully not more than a few weeks:

• Automatically detect added and removed screens. Right now it fails to draw labels on new screens. (The workaround is to reload the Hammerspoon config.)
• Add sigils to empty spaces to enable warping a window there. Algorithm yet to be determined.
• Try to ignore non-focusable windows, specifically Pop adds a window to decorate the screen that should be ignored.

[eraserhd]

CI failure appears unrelated?

[eraserhd]

@cmsj Could you review and merge this? It fixes multiple issues in WindowSigils.

In addition, I just stacked an "empty window" feature on top of this PR. I've been using this code for months, and this brings the spoon up-to-date with what I'm using and maintaining.

[eraserhd]

This also closes #238.

[cmsj]

Thanks @eraserhd - apologies for the slow response.