Ensure non-repeating Timer callback fires (#610)

CHANGELOG.md

@@ -6,6 +6,7 @@
  - Fixes aseprite constructor bug
  - Improve error handling for the onLoad function
  - Add test for child removal
+ - Fix bug where `Timer` callback doesn't fire for non-repeating timers, also fixing bug with `Particle` lifespan
 
 ## 1.0.0-rc5
  - Option for overlays to be already visible on the GameWidget

lib/timer.dart

@@ -29,6 +29,7 @@ class Timer {
       if (_current >= limit) {
         if (!repeat) {
           _running = false;
+          callback?.call();
           return;
         }
         // This is used to cover the rare case of _current being more than