qtgql 0.139.0

This release removes the previous approach of using fetch and refetch.
Instead, the user would call execute() on the operation handler.It is important to note that every time you call set_variables on the operation handler,the network layers will ignore the previous execution.

If a user wants to refetch the operation, and the variables haven't changed, one should call execute(true) again. This will force the network layers to refetch the operation.

Bug Fixes:

• Bad HTTP responses will now call the on_error callback and can be handled nythe operation handler just as if it was an operation error. Previously, failed HTTP responseswould cause the operation handler to be in a stale state, and it couldn't be used again.
• errors were not piped down to qml proxy handler. Now they are.

Contributed by ניר via PR #477