With Wayland, a window is not showing until it's shown. Until this
patch, the initial state of MetaWindow, on the other hand, was that a
window is initialized as showing. This means that for a window to
actually be classified as shown (MetaWindow::hidden set to FALSE),
something would first have to hide it.

Normally, this wasn't an issue, as normally we'd first create a window,
determine it shouldn't be visible (due to missing buffer), hide it
before the next paint, then eventually show it. This doesn't work if
mutter isn't drawing any frames at the moment (e.g. the user switched
VT), as we'd miss the hiding before showing as e result of a buffer
being attached. The most visible side effect is that a window can't be
moved as the window actor remains frozen.

This commit fixes this issue by correctly classifying a newly created
Wayland window as "hidden".

Fixes: #331