Reorganise handling of WM_SIZE to fix two generality problems.

Firstly, maximise and restore events were expected never to occur
during an interactive resize process (i.e. between WM_ENTERSIZEMOVE
and WM_EXITSIZEMOVE), but in fact Aero now allows this to happen if
you move the pointer to the top of the screen while dragging the
window.

Secondly, plain old WM_SIZE events were expected never to occur
_outside_ interactive resizes, but Aero permits that too (e.g.
Windows-left and Windows-right), and also third-party window
repositioning tools will send these.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9040 cda61777-01e9-0310-a592-d414129be87e