Allow for requesting the user's attention to the window

[TicClick]

• should be rebased onto master if eframe: Read and request window focus #2900 is merged
• add a test app to examples

calls request_user_attention() with different platform-dependent effects, as seen in UserAttentionType's documentation. the attention type enum (almost) mirrors that of winit's but I'm open to other options if this is not how it should be done.

example action (windows)

source: https://github.com/TicClick/egui-attention-tester

2023-04-16.05-04-18.mp4

[emilk]

#2900 has been merged - please rebase on it 🙏

[TicClick]

all done, thanks for the quick response

[emilk]

Looks good - it would be nice with a test of this in egui_demo_app:s backend_panel.rs - e.g. a button to request attention after a two-second delay (so the user can alt-tab away and test it properly)

[TicClick]

Looks good - it would be nice with a test of this in egui_demo_app:s backend_panel.rs - e.g. a button to request attention after a two-second delay (so the user can alt-tab away and test it properly)

can do that in the present PR, or file a new one -- what do you prefer? I have a tester app already so it won't be too much work either way

[emilk]

A separate app to test this would make sense! Please add it to examples 🙏

[emilk]

Good example, and works like a charm, but please get rid of the chrono dependency 🙏

[emilk]

Thank you!