Device support: Move processes dialog creation to settings widget.

With the process list model being publicly available from the IDevice
interface, there's no longer a need to force all subclasses to create
the dialog themselves via opaque handles.
Since clients can now check whether a device offers a process list,
the base process list model has been made abstract.

Change-Id: If4a0aa68a95b221862c287ad8397ebabe9be5909
Reviewed-by: hjk <qthjk@ovi.com>

src/plugins/debugger/gdb/startgdbserverdialog.cpp

@@ -207,7 +207,7 @@ void StartGdbServerDialog::attachToDevice()
 {
     IDevice::ConstPtr device = d->currentDevice();
     // TODO: display error on non-matching device.
-    if (!device)
+    if (!device || !device->canCreateProcessModel())
         return;
     delete d->processList;
     d->processList = device->createProcessListModel();