Show a splash screen with loading progressbar on startup if dataset/treeview is too big to be ready in less than 5 seconds about gtg HOT 3 OPEN

Closing as duplicate of #109

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Reopening/de-duplicating because until we have consistent sub-5 seconds startup / tree-building times "even with big datasets", this could in theory still be worthwhile to have (especially if the app can autodetect that "this is a big file, it'll probably take me a while to process" and only show the splash+progress launch screen when needed... but of course if we end up implementing optimizations to the point where this is no longer relevant because startup performance is great even with thousands of tasks, then we can re-close this.

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So the code could go like

if (xml file exceeds certain size, show splash screen till ready)?

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