dispatcher: Merge code from Norbert to allow to install alternative dispatchers

Norbert introduced the MockDispatcher to get nice and direct
exceptions.

Dispatcher.st

@@ -28,13 +28,24 @@ Object subclass: Dispatcher [
 
     Dispatcher class >> new [
         <category: 'private'>
-        ^ super new
-            initialize;
+        ^super new
             addToBeFinalized;
+            startDispatching;
             yourself
     ]
 
-    initialize [
+    Dispatcher class >> install [
+        <category: 'singleton'>
+        | dispatcher |
+        dispatcher := Smalltalk at: #OsmoDispatcher ifAbsentPut: [self new].
+        ^dispatcher class = self
+            ifTrue: [dispatcher]
+            ifFalse: [
+                dispatcher terminate.
+                Smalltalk at: #OsmoDispatcher put: self new]
+    ]
+
+    startDispatching [
         <category: 'private'>
         quit := false.
         queue := SharedQueue new.
@@ -66,6 +77,11 @@ Object subclass: Dispatcher [
 
         sem wait.
     ]   
+
+    terminate [
+        <category: 'private'>
+        dispatch terminate
+    ]
 ]
 
 Eval [