Make preview cursor an underline

This restores functionality from gokcehan#924
reverted in gokcehan@b47cf6d5a5, while
fixing gokcehan#1038.

Unlike before, the cursor in the preview column is now an underline
rather than a grey background. This seems to work on most terminals.
In no case should the visibility of the file name itself be affected.

For messages such as "empty" or "permission denied", the preview
cursor is the same as the active cursor for now.

There is no configuration, but there is nothing preventing adding
config as discussed in gokcehan#1038
in the future.

![Screenshot](https://user-images.githubusercontent.com/4123047/209767426-abcbddd4-965a-43a8-a5a8-6242681bbd98.png)

ui.go

@@ -433,7 +433,11 @@ func (win *win) printDir(screen tcell.Screen, dir *dir, context *dirContext, dir
 		}
 
 		if i == dir.pos {
-			st = st.Reverse(true)
+			if dirStyle.previewing {
+				st = st.Underline(true)
+			} else {
+				st = st.Reverse(true)
+			}
 		}
 
 		var s []rune