Correct CTRL+wheel zoom on touchpad

When we get fine-grained scroll events we shouldn't zoom 10% on every
event but scale zooming so they add up to 10% for every wheel click.

Task-number: QTBUG-49024
Change-Id: I08ac728bf1421148680de8fbbc76054ba2cce884
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/texteditor/texteditor.h

@@ -378,8 +378,7 @@ public:
     void circularPaste();
     void switchUtf8bom();
 
-    void zoomIn();
-    void zoomOut();
+    void zoomF(float delta);
     void zoomReset();
 
     void cutLine();