Debugger: Use a StandardRunnable for the debugger process

One step further to separate the debugger environment from the
inferior environment and to make it possible to configure a
working directory. Guessing one from the inferior's working
directory is not always a good idea.

Change-Id: I33d139c0f228ec0870556b82bc6aecca0a8e62d6
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/gdb/gdbengine.h

@@ -31,6 +31,7 @@
 #include <debugger/registerhandler.h>
 #include <debugger/watchhandler.h>
 #include <debugger/watchutils.h>
+#include <debugger/debuggeritem.h>
 #include <debugger/debuggertooltipmanager.h>
 
 #include <coreplugin/id.h>
@@ -397,13 +398,12 @@ protected:
     //
     void reloadDebuggingHelpers() override;
 
-    QString m_gdb;
-
     //
     // Convenience Functions
     //
     QString errorMessage(QProcess::ProcessError error);
     void showExecutionError(const QString &message);
+    QString failedToStartMessage();
 
     static QString tooltipIName(const QString &exp);