ProjectExplorer: Split Target and ToolRunners into smaller tasks

This increases re-usability of activities like 'port gathering',
and makes their use less dependent on actual device implementations.

Change-Id: I017cb74874f2b38c487ba2d03906a675d5618647
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/debuggerengine.h

@@ -202,11 +202,8 @@ public:
     virtual void setRunTool(DebuggerRunTool *runTool);
     DebuggerRunTool *runTool() const;
 
-    void prepare();
     void start();
 
-    void startDebugger();
-
     enum {
         // Remove need to qualify each use.
         NeedsTemporaryStop = DebuggerCommand::NeedsTemporaryStop,