BaseFileFind/FindInFiles: Clean up enabled / valid state handling

IFindFilter:  Separate enabledChanged and validChanged
SearchEngine: Add enabledChanged
GitGrep:      Enable widget based on enabled state
FindInFiles:  Update validity on search engine enabledChanged instead of
              hardcoding it to when the search path changed

Change-Id: I0c684423b871a3a4f1f164574f42e23b64cf9da0
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/git/gitgrep.h

@@ -44,7 +44,6 @@ public:
     QString title() const override;
     QString toolTip() const override;
     QWidget *widget() const override;
-    bool isEnabled() const override;
     QVariant parameters() const override;
     void readSettings(QSettings *settings) override;
     void writeSettings(QSettings *settings) const override;