For directories in preview column, dim cursor

When preview is on and consists of a directory listing, make the
cursor in there dimmer to emphasize it's not active. This makes it
visually obvious which cursor is active at all times, without
having to read the path. The active cursor is now always  the
rightmost bright one, regardless of whether the `preview` option
is on or off.

See screenshots in the pull request.

Before this, I noticed that I often open a file by accident because
I got confused which of the three cursors on the screen is the active
one.

In the future, I also hope this dimming could be used to show when an lf
window loses focus, based on @laktak's implementation of focus tracking
in gokcehan#861.

ui.go

@@ -324,7 +324,7 @@ func fileInfo(f *file, d *dir) string {
 	return info
 }
 
-func (win *win) printDir(screen tcell.Screen, dir *dir, selections map[string]int, saves map[string]bool, tags map[string]string, colors styleMap, icons iconMap) {
+func (win *win) printDir(screen tcell.Screen, dir *dir, selections map[string]int, saves map[string]bool, tags map[string]string, colors styleMap, icons iconMap, dim_cursor bool) {
 	if win.w < 5 || dir == nil {
 		return
 	}
@@ -403,6 +403,9 @@ func (win *win) printDir(screen tcell.Screen, dir *dir, selections map[string]in
 
 		if i == dir.pos {
 			st = st.Reverse(true)
+			if dim_cursor {
+				st = st.Foreground(tcell.ColorGrey)
+			}
 		}
 
 		var s []rune
@@ -855,7 +858,7 @@ func (ui *ui) draw(nav *nav) {
 
 	doff := len(nav.dirs) - length
 	for i := 0; i < length; i++ {
-		ui.wins[woff+i].printDir(ui.screen, nav.dirs[doff+i], nav.selections, nav.saves, nav.tags, ui.styles, ui.icons)
+		ui.wins[woff+i].printDir(ui.screen, nav.dirs[doff+i], nav.selections, nav.saves, nav.tags, ui.styles, ui.icons, false)
 	}
 
 	switch ui.cmdPrefix {
@@ -889,7 +892,7 @@ func (ui *ui) draw(nav *nav) {
 			preview := ui.wins[len(ui.wins)-1]
 
 			if curr.IsDir() {
-				preview.printDir(ui.screen, ui.dirPrev, nav.selections, nav.saves, nav.tags, ui.styles, ui.icons)
+				preview.printDir(ui.screen, ui.dirPrev, nav.selections, nav.saves, nav.tags, ui.styles, ui.icons, true)
 			} else if curr.Mode().IsRegular() {
 				preview.printReg(ui.screen, ui.regPrev)
 			}