Make it easier to navigate around a frame using the mouse

[bhackett1024]

There is already a way to jump between different statements in a frame using the mouse (right click -> Continue To Here, maybe some other way too, not sure). This isn't very discoverable and it would be nice to improve the source tab UI when paused in a frame so this navigation is easier. We should be trying to lure users away from the stepping buttons (and regular breakpoints, but that's a separate issue) when possible. There are a couple things here:

• Statements in the frame which are executed should be shown differently from statements which don't execute. I think that graying out code which doesn't execute and/or making it non-interactive to hover actions would work well.

• Hovering over a statement which does execute should show a jump icon somewhere, like we do with console messages. I think the icon would work well in any whitespace to the left of the executed statement. We probably can't show it when there isn't any such whitespace, but that should be pretty rare.