* modules/gui/wxwindows/*: The wxwindows interface is now a "dialogs provider" module as well as a normal "interface".
   Dialog providers expose an api that allow other modules to use the GUI dialogs the module provides (open dialogs, etc...).
   + misc small enhancements.

* modules/gui/skins/*: completely removed the dependancy on the wxwindows code (which also eliminates the need for the basic_skins module).
   The skins module will now try to load a "dialogs provider" to display the open, messages, file info and preferences dialogs.