transaction: Add ready signal

This signal is emitted after all the added operations and their dependencies
are resolved and we have the full list of things that will be
done as part of the transaction. At this point you can call
flatpak_transaction_get_operations() and decide if you want to
continue with the operation.

Closes: #1797
Approved by: alexlarsson
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f