Browser: Port browser to GML compilation

[overriden-sfdd]

This PR contributes to #20518
It ports Browser. I'm open for suggestions regarding refactoring of BrowserTabWidget. It's quite puzzling. I'm not sure the way I did it is the best. Please review carefully.
Additionally, I got confused with all the different initialization functions like try_create, initialize, and setup. Do we prefer initialize over setup now?

[overriden-sfdd]

Oh yes, one more thing to add that felt kinda wrong. The GML compiler works in a way that prevents forward declaration in widget headers. I wonder if there's anything we can do about that and if that's intended. Widget headers become quite huge because of that.

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[nico]

Looks like this needs rebasing now, sorry!

As for the questions, iirc @kleinesfilmroellchen wrote the GML compiler. Film, do you know what we do about forward declarations? Just not worry about it?