fix special key handling in edit controls

This fixes some special key handling in edit controls after #5008.

The first commit brings back the possibility to use `Backspace` in normal edit controls without jumping back out of the dialog or window. @Memphiz: `CGUIEditControl::m_edit` is only used by the special OSK handling on OSX/IOS so could you give this a try and see if everything is still working?

The second commit brings back support for `Home` and `End` when being in the OSK.