refactor: optional extra ring around marker

[robertwidfen]

See issue #243.

Make markers smaller and draw additional ring only if Alt is held. Marker is commited at button release and thus stays moveable.

[RobertMueller2]

Code looks good, function looks good.

Just two minor things I noticed:

• when dragging the marker and pressing any key which is not bound, ring is toggled once
• when dragging the marker and pressing any key which is a tool shortcut, there's no redraw and the marker remains until next redraw (which might not happen until next tool action). This isn't exclusive to marker though, and perhaps we need to fix it outside of this PR. Not sure if it broke recently, or nobody ever looked close enough :)

[robertwidfen]

Thanks for testing 🤗 - indeed the code was odd, I checked for the Alt-modifier where I should check for the keys Alt_L and Alt_R - at a second glance I added a "!" where it did not make sense.

The tool key issue is actually magically fixed in https://github.com/robertwidfen/Satty/tree/feat_generic_key_bindings #450 - first I was not able to reproduce it as I often work on a merge or all branches and the jj absorb fixes. 🙃 But I want to test this a bit more before creating a pull request.

It is still there, had StopPropagation for some minutes in the code, but this also stops tool switching - created #540 for discussion of expected behavior.

[RobertMueller2]

lgtm.