Improve egui support

[kirdaybov]

Summary

Future improvements

Add support for text input, not just key codes

At the moment text input is handled per key code, disregarding the keyboard layout used and modifier keys pressed

Add support for missing egui inputs

Zoom, copy, file drop and some other nice to have inputs that egui knows how to react to

Save state of the egui when closing the application

Not supported by the lib by default

Clip rects are not used

Returned by tesselate clip rects are not used at the moment.

Handle output

end_frame() returns output which holds some information that can be used. For example it returns an icon type that we can set to match egui state.

Previous work

• [x] #487