Add "load init file" after initial setup

[tobyclh]

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name: Add load init file about: Expose the underlying imgui function that allow loading of init file after initial setup
assignees: ''

Description:

Currently DPG doesn't allow the user to load the init file after setting up the viewport, this PR exposes the imgui function, this is a very small change that I hope we can squeeze in so we can use the upstream version which updates all the backend libraries.

Concerning Areas:

This is a very small change so I don't see it breaking anything immediately. But I have only tested this on Windows and Mac, and I don't have a linux machine with me atm, but I can sort it out if required.