Highlighting trailing whitespaces

Added option `hltrailingws` for highlighting trailing whitespaces
at the end of lines. Note that it behaves in a "smart" way.
It doesn't highlight newly added (transient) trailing whitespaces
that naturally occur while typing text. It would be annoying to
see transient highlighting every time we enter a space at the end
of a line while typing.
So a newly added trailing whitespace starts being highlighting
only after the cursor moves to another line. Thus the highlighting
serves its purpose: it draws our attention to annoying sloppy
forgotten trailing whitespaces.

internal/action/bufpane.go

@@ -509,6 +509,13 @@ func (h *BufPane) HandleEvent(event tcell.Event) {
 			InfoBar.ClearGutter()
 		}
 	}
+
+	cursors := h.Buf.GetCursors()
+	for _, c := range cursors {
+		if c.NewTrailingWsY != c.Y {
+			c.NewTrailingWsY = -1
+		}
+	}
 }
 
 // Bindings returns the current bindings tree for this buffer.