Previously app code was run from within the event loop
which lead to file dialogs (e.g. using nfd2) to hang
(see https://github.com/rust-windowing/winit/issues/1779)
Now egui_glium polls for events and then runs the app code.
* Use old 1.51 toolchain instead of bleeding edge
1.52 and 1.53 has problems with incremental compilation,
so some people chose to stay on 1.51 for now.
So let's make sure egui supports 1.51 for a while!
* Update to cint 0.2.2 to get rust 1.51.0 compatability
* Deny doc errors
* Add intentional bad intradoc link to test CI
* Add cargo doc to CI
* Fix carg doc web (add wasm32 target)
* Fix intentionally broken doc-link
