Geary.Imap: Make command cancellable a property of the command object

Since both submitting a command no longer requires a cancellable, and it
is desirable to avoid sending a queued command that has already been
cancelled beforehand, add a new `Command.should_send` Cancellable
property to specify if a command should (still) be sent or not, and stop
passing a cancellable to ClientSession when submitting commands.

Allow call sites to pass in existing cancellable objects, and thus also
add it it as a ctor property to the Command class and all subclasses.

Lastly, throw a cancelled exception in `wait_until_complete` if send
was cancelled so that the caller knows what happened.

Remove redundant cancellable argument from
`Imap.Client.command_transaction_async` and rename it to
`submit_command` to make it more obvious about what it does.

test/engine/imap/command/imap-create-command-test.vala

@@ -16,7 +16,7 @@ class Geary.Imap.CreateCommandTest : TestCase {
 
     public void basic_create() throws Error {
         assert_equal(
-            new CreateCommand(new MailboxSpecifier("owatagusiam/")).to_string(),
+            new CreateCommand(new MailboxSpecifier("owatagusiam/"), null).to_string(),
             "---- create owatagusiam/"
         );
     }
@@ -25,7 +25,8 @@ class Geary.Imap.CreateCommandTest : TestCase {
         assert_equal(
             new CreateCommand.special_use(
                 new MailboxSpecifier("Everything"),
-                ALL_MAIL
+                ALL_MAIL,
+                null
             ).to_string(),
             "---- create Everything (use (\\All))"
         );