[REG 5.15->6.3] Multiple context menus can be opened with synthetic touch events

Touchbrowser is a manual test application for emulating touch events by mouse.
After upgrading it to Qt6, multiple context menus can be opened, which does not happen on a real touch device.

Steps:

• Press a mouse button for at least one second, then release it,
• Move the cursor out of the context menu,
• Then press the same mouse button for at least one second and release it.
   
  A second context menu appears, but the first one will not be closed.
   
  Even if only one context menu is opened, its MenuItems do not react to clicks. When anything else is clicked in the WebEngineView, the context menu does not disappear, but it should.

Workaround: use another mouse button for the next click after triggering the context menu.