Restrict cursorline to editing area

[ChrHorn]

The newly merged cursorline currently is drawn across the whole window, including the gutter. I think it would look better if the cursorline is only drawn in the editing area, that's how it's done in VSCode, Vim, etc. We also already can set the background (for the line numbers) with ui.linenr.selected, so it's a bit redundant.

[archseer]

I think this is a s side-effect because the code was copied from the debugger, where we did want to highlight the whole line.