move find usages to TextEditor

In preperation for supporting find usages by the language client plugin

Task-number: QTCREATORBUG-21577
Change-Id: I7a6da3a9d53478c1d486e0ddc5829c9ea09a2a20
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/qmljseditor/qmljseditor.h

@@ -74,7 +74,7 @@ public:
 
     void inspectElementUnderCursor() const;
 
-    void findUsages();
+    void findUsages() override;
     void renameUsages();
     void showContextPane();