Redesign "select signing key" api dialog

OpenKeychain/src/main/res/menu/select_identity_menu.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<menu xmlns:android="http://schemas.android.com/apk/res/android">
+    <item
+        android:id="@+id/list_all_keys"
+        android:title="List all keys"
+        android:checkable="true"
+        android:orderInCategory="1" />
+</menu>