Highlight hovered GraphEdit connection by widening the line

[Sauermann]

Currently, GraphEdit connections are highlighted by tinting the color. With this change, the connections are highlighted by widening the line by a configurable factor.

The current method of highlighting connections has the following two properties:

1. The tint color is global for all connections, so when different colors are used, some of them can be very similar to the tint color, so that there is nearly no visual feedback for some highlighted connections.
2. If the tint color is darker than the connection color, the connection will be displayed darker while hovering. So while hovered It takes a back seat visually, even though the hovering should actually emphasize the connection.

Replacing the tint color by a configurable theme option solves these issues and also additionally allows users to change the settings in a way, that ho highlight happens.

While this PR changes the theme-API and is therefore technically compatibility breaking, the GraphEdit node is still marked as experimental, so this should not be a deal-breaker. This is no longer an issue, since theme properties are no longer removed.

CC @Geometror

Updated 2024-12-18: Set default values to 0%.

[Geometror]

While I like this idea, and it fits pretty good in the VisualShader editor, I'm against removing the tint color.
If someone doesn't like highlighting via color, they can just set the tint color to a color with an alpha value of 0.
By setting the default value of connection_hover_thickness to 0%, we wouldn't even need to break compatibility.

[Sauermann]

Sounds good. I have removed the removal of the tint color.

[Geometror]

Even if GraphEdit is experimental, I'd suggest using 0% as a default value for the widening, as we would still kind of break compatibility (in the manner of a changed default visual appearance that some users might not prefer).
Adjusting the editor theme should be done in a subsequent PR then.

[Repiteo]

Thanks!