added filepicker to load rust engine.

[disordinary]

Hi Sineaggi,

I'd like to contribute to this project. To make things easier in the future I just added a quick and simple file picker to select the xi-editor core, it stores this in the registry.

In the future the xi-editor would of course be bundled with the application in install and the registry key won't be necessary, however for now it's a good solution to allow multiple people to work on the codebase.

I hope that's okay.

[disordinary]

It took quite a few days to merge, are you interested in collaborating on a windows version? I've made a start of going it alone but would be happy to look at moving the code into your repository. Many hands, etc.

[Sineaggi]

Hey, yes, sorry for not responding in a reasonable manner, things have been crazy on my side.

Honestly, I'd prefer not to have to modify the registry and simply assume that that xicore executable is in the same directory, similar to how the swift frontend operates.
I've been thinking about writing a custom build script run a cargo build and copy the resulting binary into the build folder, I just haven't gotten around to it yet.

I'm going to keep this open before I commit to a registry variable, but for the time being I'll push a fix that assumes the xicore executable is in the same directory as the gui executable.

[Sineaggi]

I've pushed a quick 'fix' in 78ab5da.