Support for multiple windows

[jakobhellermann]

builds on top of #12

Diff only to #12

Changes:

• &egui_context.ctx -> egui_context.ctx()
• new egui_context.ctx_for_window(window_id) for drawing to another window
• egui::setup_pipeline and egui::RenderGraphConfig which needs to get run for additional windows
• Egui{Shapes, Output, Input} -> HashMap<WindowId, Egui{Shapes, Output, Input}>

Let me know what you think 😄

[mvlabat]

This is absolutely awesome! It was in my plans to add multiple windows support, but you took care of it much earlier than I could. :) Thanks so much for working on this!

I'll take a closer look a bit later.. And I'll probably procrastinate it for a while until we merge 0.5.

[mvlabat]

Thanks a lot for your contributions!

I've just published 0.4. :)