Added locking to WinDOS backend

Could not find any usefull information on WinAPI thread safety.
Simply assumed WinDOS functions aren't thread safe, to avoid any
trouble to begin with.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Exposed function to generate expose event for window

 - Function takes rect and generates expose event
 - Used by subview widget to implement "refresh" function

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>