[Development] QtWayland platform themes

[Pier Luigi]

Hi,

I was running some applications on the qwindow-compositor example and
I noticed that no icons are loaded from the icon theme, in fact it's
the platform theme plugin not being loaded at all.

Looking at QGuiApplication I have seen how loading works:

1. Ask the platform integration for a list of names. Currently the
wayland plugin doesn't implement it, so it returns an empty
QStringList.
2. Look for a theme plugin. It can't look at any plugin because it
iterates an empty list returned by the default themeNames().
3. Fallback on the null theme.

The xcb plugin implements themeNames() and createPlatformTheme() using
QGenericUnixTheme.
Is there a reason why the Wayland plugin is not doing the same?

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