syncthing: have tray wait for bar to load

[Jdogzz]

Description

Add --wait flag to tray to avoid loading before bar. Attempts to fix #3416 per the comments. It appears that #4548 already has some work towards this, but that's been sitting stale so this present and trivial PR is a just to tide us over until that gets sorted out.

Checklist

• Change is backwards compatible.

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