[RFC 2/2] Share user32 combo with comctl32 combo

[Gabriel Ivăncescu]

Hi Fabian,

I've been thinking about this a bit, and cleaner would be to include a
.c file but in its proper directory (so it gets compiled by itself).

For example, say in user32 we have the large implementation (with
#ifdefs) of combo.c, at the top it has something like:

#ifndef CONTROL_VERSION
#define CONTROL_VERSION 1
#endif

And gets compiled as normal. Then in comctl32's combo.c we'd have
something like:

#define CONTROL_VERSION 2
#include "../user32/combo.c"

Seems cleaner to me.