When clicking on file preview, only open files on right-click

Currently, when mouse is on, I often click on the preview accidentally
since it's such a large target. Moreover, there is very little space I
can click without having the `lf` state change drastically (since I must
click on the current file for the preview to stay the same).

This commit prevents that.

doc.go

@@ -218,10 +218,10 @@ The following additional keybindings are provided by default:
 If the 'mouse' option is enabled, mouse buttons have the following default effects:
 
 	Left mouse button
-	    Click on a file or directory to select it. To open a file, click on the preview.
+	    Click on a file or directory to select it.
 
 	Right mouse button
-	    Enter a directory or open a file.
+	    Enter a directory or open a file. Also works on the preview window.
 
 	Scroll wheel
 	    Scroll up or down.

lf.1

@@ -247,12 +247,12 @@ If the 'mouse' option is enabled, mouse buttons have the following default effec
 .PP
 .EX
     Left mouse button
-        Click on a file or directory to select it. To open a file, click on the preview.
+        Click on a file or directory to select it.
 .EE
 .PP
 .EX
     Right mouse button
-        Enter a directory or open a file.
+        Enter a directory or open a file. Also works on the preview window.
 .EE
 .PP
 .EX

ui.go

@@ -1209,6 +1209,9 @@ func (ui *ui) readNormalEvent(ev tcell.Event, nav *nav) expr {
 			if err != nil {
 				return nil
 			} else if !curr.IsDir() || gOpts.dirpreviews {
+				if tev.Buttons() != tcell.Button2 {
+					return nil
+				}
 				return &callExpr{"open", nil, 1}
 			}
 			dir = ui.dirPrev