gmountsource: Fix deadlocks in synchronous API
Synchronous GMountSource API is implemented using the asynchronous API with help of mutexes and conditions. But it seems that it is not guaranteed that GAsyncReadyCallback is called on another thread and thus this solution may cause deadlocks consequently. Let's rather use solution based on custom mainloop to prevent potential deadlocks. Alternatively, we could use synchronous API generated by gdbus-codegen to implement this functionality, but the solution with custom mainloop seems to be less error-prone.
Closes: #383 (closed)