io/key: change Modifiers.String separator to "-"

We're about the express sets of key combinations as <modifiers>-<keys>
where modifiers are separated by dashes as well. To make Modifers.String
useful for expressing key sets, change its separator to "-".

Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/key/key.go

@@ -276,7 +276,7 @@ func (m Modifiers) String() string {
 	if m.Contain(ModSuper) {
 		strs = append(strs, NameSuper)
 	}
-	return strings.Join(strs, "|")
+	return strings.Join(strs, "-")
 }
 
 func (s State) String() string {