w32_common: fix mouse clicks

Apparently this has been broken for a year or so. The were three
reasons for the breakage here:

    1. The window dragging hack prevented any DOWN event from
       passing through since it always returned before we even got
       the button.
    2. The window style had CS_DBLCLKS in its flags, so we did not
       get any DOWN events when the OS had detected a double click
       (instead expecting us to handle a DBL event).
    3. We never sent any mouse buttons when mouse movement handling
       was disabled.