Implement moving cursor up/down within a wrapped line

Modified behavior of CursorUp, CursorDown, CursorPageUp etc:
if softwrap is enabled, cursor moves by visual lines, not logical lines.

TODO: implement it also for Home and End keys: move cursor to the
visual start or end of a line. I haven't implemented it for now, because
I'm not sure what should be the behavior of StartOfTextToggle then
(considering that Home key is bound to StartOfTextToggle by default).

Fixes #1598

internal/buffer/cursor.go

@@ -67,6 +67,10 @@ func (c *Cursor) GotoLoc(l Loc) {
 
 // GetVisualX returns the x value of the cursor in visual spaces
 func (c *Cursor) GetVisualX() int {
+	if c.buf.GetVisualX != nil {
+		return c.buf.GetVisualX(c.Loc)
+	}
+
 	if c.X <= 0 {
 		c.X = 0
 		return 0