Add Geary.Account.is_online property

This will be useful for displaying this status to the user in the
future, and allows removing MinimalFolder.is_remote_available, which is
hopelessly innacturate in the face of network problems.

test/engine/api/geary-account-mock.vala

@@ -31,6 +31,8 @@ public class Geary.MockAccount : Account, MockObject {
     }
 
 
+    public override bool is_online { get; protected set; default = false; }
+
     protected Gee.Queue<ExpectedCall> expected {
         get; set; default = new Gee.LinkedList<ExpectedCall>();
     }