Fix crash with blocking Open dialog

QDialog::exec() is bad and should not be used unless strictly necessary.
The exec() call is blocking and anything can happen before it returns to the caller.

The old workaround is to use a QPointer to guard the dialog.
Yet there is a crash when calling QCoreApplication::quit() via dbus while the dialog is open.

The proper fix is to call the dialog in a non-blocking way. Nowadays this is trivial thanks to C++11 lambdas.

Differential Revision: D1786