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/buffer/cursor.go

@@ -30,6 +30,11 @@ type Cursor struct {
 	// to know what the original selection was
 	OrigSelection [2]Loc
 
+	// The line number where a new trailing whitespace has been added
+	// or -1 if there is no new trailing whitespace at this cursor.
+	// This is used for checking if a trailing whitespace should be highlighted
+	NewTrailingWsY int
+
 	// Which cursor index is this (for multiple cursors)
 	Num int
 }
@@ -38,6 +43,8 @@ func NewCursor(b *Buffer, l Loc) *Cursor {
 	c := &Cursor{
 		buf: b,
 		Loc: l,
+
+		NewTrailingWsY: -1,
 	}
 	c.StoreVisualX()
 	return c