QAccessiblePlugin class and QAccessible.installFactory() static method should be included in PySide

The QAccessiblePlugin class and the installFactory() static method of QAccessible class are not included in PySide. They're required for implementing accessibility for controls, along with other dependancies.

As a result, developers cannot implement accessibility for custom widgets or customize the accessibility information of standard widgets. In fact, I'm facing some issues while developing an accessible application using the standard widgets and I'm blocked from customizing the accessibility for them in order to comply with accessibility requirements and guidelines.