feat(app): event line provides entire line contents as a single event; add(event): .cancel event

Introduce `.cancel` event that is fired when the user sends EOF
in *non raw mode* renderings. The event `.line` now sends the
entire line (even if larger than the internal buffer) but requires
now an `Allocator`.

src/event.zig

@@ -8,6 +8,11 @@ pub const SystemEvent = union(enum) {
     init,
     /// Quit event to signify the end of the event loop (rendering should stop afterwards)
     quit,
+    /// Cancel event to signify that the user provided an EOF
+    ///
+    /// Usually this event is only triggered by the system in *non raw mode*
+    /// renderings otherwise the corresponding `.key` event would be fired instead.
+    cancel,
     /// Resize event to signify that the application should re-draw to resize
     ///
     /// Usually no `Container` nor `Element` should act on that event, as it
@@ -23,7 +28,10 @@ pub const SystemEvent = union(enum) {
         /// associated error message
         msg: []const u8,
     },
-    /// Input line event received in non *raw mode* (instead of individual `key` events)
+    /// Input line event received in *non raw mode* (instead of individual `key` events)
+    ///
+    /// This event contains the entire line until the ending newline character
+    /// (which is included in the payload of this event).
     line: []const u8,
     /// Input key event received from the user
     key: Key,