Debugger: Add toolchain combos to start dialogs, refactor detection.

Add combo box listing toolchains with debuggers to start external,
attach and core.
Another attempt at streamlining engine detection:
Split in detection functions that first collect a list of
available engines by preference, then remove disabled and
wrongly configured engines and use the remaining best.
matching. checkconfiguration is now the central place where
engine detection and config check takes place.

Rubber-stamped-by: hjk

src/plugins/debugger/debuggerdialogs.h

@@ -54,6 +54,9 @@ class StartRemoteEngineDialog;
 
 QT_END_NAMESPACE
 
+namespace ProjectExplorer {
+class Abi;
+}
 
 namespace Debugger {
 namespace Internal {
@@ -82,7 +85,15 @@ public:
     QString executableFile() const;
     QString coreFile() const;
 
+    ProjectExplorer::Abi abi() const;
+    void setAbi(const ProjectExplorer::Abi &);
+
+private slots:
+    void changed();
+
 private:
+    bool isValid() const;
+
     Ui::AttachCoreDialog *m_ui;
 };
 
@@ -97,6 +108,9 @@ public:
     qint64 attachPID() const;
     QString executable() const;
 
+    ProjectExplorer::Abi abi() const;
+    void setAbi(const ProjectExplorer::Abi &);
+
     virtual void accept();
 
 private slots:
@@ -159,8 +173,16 @@ public:
     QString workingDirectory() const;
     void setWorkingDirectory(const QString &str);
 
+    ProjectExplorer::Abi abi() const;
+    void setAbi(const ProjectExplorer::Abi &);
+
     bool breakAtMain() const;
 
+    bool isValid() const;
+
+private slots:
+    void changed();
+
 private:
     Ui::StartExternalDialog *m_ui;
 };