NME Preview window does not pop out correctly

[RaananW]

Fixes #16569
Adding onMounted functionality for the preview so that we know when it is mounted and when we can run the next init

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Snapshot stored with reference name:
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Links to test babylon tools with this snapshot:

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Visualization tests for WebGPU

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WebGL2 visualization test reporter:

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You have made possible changes to one or more graph tools. To test the snapshots:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/TOOLS/refs/pull/16571/merge/nodeEditor/
https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/TOOLS/refs/pull/16571/merge/nodeGeometryEditor/
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https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/TOOLS/refs/pull/16571/merge/guiEditor/

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WebGL2 visualization test reporter:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16571/merge/testResults/webgl2playwright/index.html

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Visualization tests for WebGPU

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16571/merge/testResults/webgpuplaywright/index.html