VCS: Introduce Base class for VCS plugins, use in git.

Fixes:
- Cannot diff a file that does not belong to a project
- Cannot commit when a temporary diff/log view is open
  due to the current file pointing to a temporary directory
- git's project-related actions not passing the correct
  relative path.

Implementation:
- Centralize code to listen for Qt Creator's relevant state changes
  in VCSBasePlugin, dispatching the changes to the instances affected.
  (avoiding multiple invocations of searches/QFileInfo on current).
- Do the same for the corelistener catching closing SubmitEditors.
- Introduce VCSBasePluginState representing the relevant state
  (current file/project).
- Call git with working directory set and relative arguments
- Remove setEnabled/isEnabled() logic of IVersionControl
- Pass toplevel from VCSManager to avoid duplicate searches.

src/plugins/git/changeselectiondialog.h

@@ -37,21 +37,22 @@
 namespace Git {
 namespace Internal {
 
-class GitPlugin;
-
 class ChangeSelectionDialog : public QDialog
 {
     Q_OBJECT
 public:
     ChangeSelectionDialog(QWidget *parent = 0);
 
+    QString change() const;
+
+    QString repository() const;
+    void setRepository(const QString &s);
+
 public slots:
     void selectWorkingDirectory();
 
-private:
-    friend class GitPlugin;
+private:    
     Ui_ChangeSelectionDialog m_ui;
-
 };
 
 } // namespace Internal