Add message details menu button

Adds a "Properties" menu button in conversation, when one message
is selected, which displays details about the message:
- Type (SMS or MMS)
- Sender phone number (or receiver if it is a sent message)
- Used SIM
- Date sent at
- Date received at (if it is an incoming message)

This closes #19

app/src/main/res/values-es/strings.xml

@@ -47,6 +47,13 @@
     <string name="schedule_send_warning">Mantenga el teléfono encendido y asegúrese de que no hay nada que mate la aplicación en segundo plano.</string>
     <string name="update_message">Actualizar mensaje</string>
     <string name="send_now">Enviar ahora</string>
+    <!-- Message details -->
+    <string name="message_details">Message details</string>
+    <string name="message_details_sender">Sender</string>
+    <string name="message_details_receiver">Receiver</string>
+    <string name="message_details_type">Type</string>
+    <string name="message_details_sent_at">Sent at</string>
+    <string name="message_details_received_at">Received at</string>
     <!-- Notifications -->
     <string name="channel_received_sms">Mensaje recibido</string>
     <string name="new_message">Nuevo mensaje</string>
@@ -110,4 +117,4 @@
         Haven't found some strings? There's more at
         https://github.com/SimpleMobileTools/Simple-Commons/tree/master/commons/src/main/res
     -->
-</resources>
+</resources>