Rename gui.__Handle back to gui.Hwnd.
Change Gui lifetime management (suggested by Lexikos). Details below:

- The global Gui list now increases the reference count of the Gui.
- The extra sink ownership parameter of GuiCreate() has been removed.
- When the Gui is automatically canceled in MsgSleep(); if there is
  only one outstanding reference to it (that is, belonging to the
  global Gui list); the Gui is destroyed.
- Script::~Script() now destroys any remaining Guis for safety reasons.