Add ViewportCommand::RequestCut, RequestCopy and RequestPaste to trigger Clipboard actions (#4035)

### Motivation
We want to offer our users a context menu with `Cut`, `Copy` and `Paste`
actions. `Paste` is not possible out of the box.

### Changes
This PR adds `ViewportCommand::RequestCut`,
`ViewportCommand::RequestCopy` and `ViewportCommand::RequestPaste`. They
are routed and handled after the example of
`ViewportCommand::Screenshot` and result in the same code being executed
as when the user uses `CTRL+V` style keyboard commands.

### Reasoning
In our last release we used an instance of
`egui_winit:📋:Clipboard` in order to get the `Paste`
functionality.

However Linux users on Wayland complained about broken clipboard
interaction (https://github.com/mikedilger/gossip/issues/617). After a
while of digging I could not find the issue although I have found
references to problems with multiple clipboards per handle before
(https://gitlab.gnome.org/GNOME/mutter/-/issues/1250) but I compared
mutter with weston and the problem occured on both.

So to solve this I set out to extend egui to access the clipboard
instance already present in egui_winit. Since there was no trivial way
to reach the instance of `egui_winit::State` I felt the best approach
was to follow the logic of the new `ViewportCommand::Screenshot`.

### Variations
It could make sense to make the introduced `enum ActionRequested` a part
of crates/egui/src/viewport.rs and to then wrap them into one single
`ViewportCommand::ActionRequest(ActionRequested)`.

### Example
```Rust
let mut text = String::new();
let response = ui.text_edit_singleline(&mut text);
if ui.button("Paste").clicked() {
    response.request_focus();
    ui.ctx().send_viewport_cmd(ViewportCommand::RequestPaste);
}
```

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_glow/src/winit.rs

@@ -1,7 +1,7 @@
 pub use egui_winit;
 pub use egui_winit::EventResponse;
 
-use egui::{ViewportId, ViewportOutput};
+use egui::{ahash::HashSet, ViewportId, ViewportOutput};
 use egui_winit::winit;
 
 use crate::shader_version::ShaderVersion;
@@ -79,17 +79,17 @@ impl EguiGlow {
             log::warn!("Multiple viewports not yet supported by EguiGlow");
         }
         for (_, ViewportOutput { commands, .. }) in viewport_output {
-            let mut screenshot_requested = false;
+            let mut actions_requested: HashSet<egui_winit::ActionRequested> = Default::default();
             egui_winit::process_viewport_commands(
                 &self.egui_ctx,
                 &mut self.viewport_info,
                 commands,
                 window,
                 true,
-                &mut screenshot_requested,
+                &mut actions_requested,
             );
-            if screenshot_requested {
-                log::warn!("Screenshot not yet supported by EguiGlow");
+            for action in actions_requested {
+                log::warn!("{:?} not yet supported by EguiGlow", action);
             }
         }