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/gitversioncontrol.h

@@ -46,9 +46,6 @@ public:
 
     virtual QString name() const;
 
-    virtual bool isEnabled() const;
-    virtual void setEnabled(bool enabled);
-
     bool managesDirectory(const QString &directory) const;
     virtual QString findTopLevelForDirectory(const QString &directory) const;