PhoshLayerShell: Don't realize layer shell's right away

This allows us to create them early and keep them around. It also
makes them behave more like regular Gtk widgets.

Closes: #91

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
5 jobs for wip/layer-surface-realize in 4 minutes and 46 seconds (queued for 2 seconds)
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librem5
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librem5
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librem5
test:smoke:one-output

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librem5
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