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/display/bufwindow.go

@@ -495,7 +495,11 @@ func (w *BufWindow) displayBuffer() {
 			vloc.X = w.gutterOffset
 		}
 
-		leadingwsEnd := len(util.GetLeadingWhitespace(b.LineBytes(bloc.Y)))
+		bline := b.LineBytes(bloc.Y)
+		blineLen := util.CharacterCount(bline)
+
+		leadingwsEnd := len(util.GetLeadingWhitespace(bline))
+		trailingwsStart := blineLen - util.CharacterCount(util.GetTrailingWhitespace(bline))
 
 		line, nColsBeforeStart, bslice, startStyle := w.getStartInfo(w.StartCol, bloc.Y)
 		if startStyle != nil {
@@ -532,6 +536,25 @@ func (w *BufWindow) displayBuffer() {
 						}
 					}
 
+					if b.Settings["hltrailingws"].(bool) {
+						if s, ok := config.Colorscheme["trailingws"]; ok {
+							if bloc.X >= trailingwsStart && bloc.X < blineLen {
+								hl := true
+								for _, c := range cursors {
+									if c.NewTrailingWsY == bloc.Y {
+										hl = false
+										break
+									}
+								}
+								if hl {
+									fg, _, _ := s.Decompose()
+									style = style.Background(fg)
+									dontOverrideBackground = true
+								}
+							}
+						}
+					}
+
 					for _, c := range cursors {
 						if c.HasSelection() &&
 							(bloc.GreaterEqual(c.CurSelection[0]) && bloc.LessThan(c.CurSelection[1]) ||