Add Key::Cut, Key::Copy, Key::Paste (#3725)

The comment added in commit 8a0bc97e ("[egui_glium] Fix paste") seems to
assume that `winit` "should have translated" common "paste" keyboard
combos to a `Cut`/`Copy`/`Paste` "KeyCode", but completely glossed over
the fact that this `KeyCode` (now also `NamedKey`) maps to a special key
dedicated to this purpose found on some keyboards and OSes. Make sure
that this key is still handled in addition to the combo that is
detected.

---

Note that this PR does not compile as it is (and I have hence not tested
this nor even ran into this limitation), just noticed this inconsistency
while failing to understand a code comment. We'd have to decide if the
variants should be added to `egui::Key` or if these helper functions
need to take `winit` keys (`ScanCode` or `NamedKey`) directly?

I should have an old keyboard with a physical paste key in a drawer
somewhere. And on Android there are special copy/paste events that can
be sent by a virtual keyboard or the debug shell, so that this can be
properly tested before it is merged.
(Except that the current `clipboard` implementation is not supported on
Android)

crates/egui/src/data/input.rs

@@ -879,6 +879,10 @@ pub enum Key {
     PageUp,
     PageDown,
 
+    Copy,
+    Cut,
+    Paste,
+
     // ----------------------------------------------
     // Punctuation:
     /// `:`
@@ -1005,6 +1009,9 @@ impl Key {
         Self::End,
         Self::PageUp,
         Self::PageDown,
+        Self::Copy,
+        Self::Cut,
+        Self::Paste,
         // Punctuation:
         Self::Colon,
         Self::Comma,
@@ -1102,6 +1109,10 @@ impl Key {
             "PageUp" => Self::PageUp,
             "PageDown" => Self::PageDown,
 
+            "Copy" => Self::Copy,
+            "Cut" => Self::Cut,
+            "Paste" => Self::Paste,
+
             "Colon" | ":" => Self::Colon,
             "Comma" | "," => Self::Comma,
             "Minus" | "-" | "−" => Self::Minus,
@@ -1209,6 +1220,10 @@ impl Key {
             Key::PageUp => "PageUp",
             Key::PageDown => "PageDown",
 
+            Key::Copy => "Copy",
+            Key::Cut => "Cut",
+            Key::Paste => "Paste",
+
             Key::Colon => "Colon",
             Key::Comma => "Comma",
             Key::Minus => "Minus",