Could not type special characters on non-english keyboards

On german/french/spanish keyboards, typing [ requires modifier
keys like AltGr, so the `mod==0` condition is wrong.

Fixes #2573

ch != 0 is useless because IsPrint is implemented this way:
	if uint32(r) <= MaxLatin1 {
		return properties[uint8(r)]&128 != 0
	}
with properties[0] set to 1 (so, bit 7 not set)
-> 0 is not printable.

pkg/gui/editors.go

@@ -47,8 +47,7 @@ func (gui *Gui) handleEditorKeypress(textArea *gocui.TextArea, key gocui.Key, ch
 	case key == gocui.KeyCtrlY:
 		textArea.Yank()
 
-		// TODO: see if we need all three of these conditions: maybe the final one is sufficient
-	case ch != 0 && mod == 0 && unicode.IsPrint(ch):
+	case unicode.IsPrint(ch):
 		textArea.TypeRune(ch)
 	default:
 		return false