Creating wxPaintDC for a window outside of any wxEVT_PAINT handler
already resulted in assert failures and crash due to using the empty
wxDidCreatePaintDC stack, but the assert message was not really clear,
so improve it by stating explicitly that wxPaintDC can only be created
from wxEVT_PAINT handlers.
Also check that wxPaintDC is being created for the correct window: this
wasn't detected at all before, but could still result in a lot of grief,
so check for this too.
Finally, create a new private header with the paint data stack variable
declaration instead of using "extern" to declare it manually in wxDC
code.
