For asynchronous dialogs, use the topmost modal shell

For asynchronous dialogs, use the topmost modal shell

Push and fetch results are shown in dialogs that
* are shown asynchronously, and
* moreover are triggered from inside jobs.

I don't like jobs throwing dialogs at the user anyway, but I can see
the desire here to run the potentially long remote operation in a job.
Still, the PushToGerritPage does it without job...

At the very least, such asynchronous dialogs, whether or not they
themselves are modal, must use the topmost modal shell as parent. Using
a parent, like the active window's shell, that already has a modal
child may lock up the application.

Bug: 487209
Change-Id: I460e625051e48ecedab0db7191a6dd8846dacc7d
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>