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/buffer.go

@@ -196,6 +196,12 @@ type Buffer struct {
 	// the buffer module cannot directly call the display's API (it would mean
 	// a circular dependency between packages).
 	OptionCallback func(option string, nativeValue interface{})
+
+	// The display module registers its own GetVisualX function for getting
+	// the correct visual x location of a cursor when softwrap is used.
+	// This is hacky. Maybe it would be better to move all the visual x logic
+	// from buffer to display, but it would require rewriting a lot of code.
+	GetVisualX func(loc Loc) int
 }
 
 // NewBufferFromFileAtLoc opens a new buffer with a given cursor location

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