Investigate activating windows of already active application

On Wayland we claim that we don't support window activation today (QPlatformIntegration::WindowActivation). But there are two cases for window activation:

• Activating the application from being in the background
• Activating a window of an already active application

In our tests we often use requestActivate() for the latter case.

We already implement QWaylandWindow::raise(), so changing the window level is already possible. It seems strange that the Wayland protocol would prevent an app from changing keyboard focus from one window to another of its already active application, so this might be something we can support, even if don't support making the application active.