Show timestamp in expanded chat

When the last message is older than SCROLLBACK_IMMEDIATE_TIME (1
minutes), show a timestamp in the middle, indicating the time it
was sent.
Use the same style for presence changes, but show them on the left.

https://bugzilla.gnome.org/show_bug.cgi?id=617228

data/theme/gnome-shell.css

@@ -957,6 +957,13 @@ StTooltip StLabel {
     border-radius: 4px;
 }
 
+.chat-meta-message {
+    padding-left: 4px;
+    border-radius: 4px;
+    font-size: 14px;
+    color: #bbbbbb;
+}
+
 .chat-response {
     padding: 4px;
     border-radius: 4px;