More Header Cleanups

In this edition of Header Cleanups:

* Remove global Windows.h include
* Precompiled headers for the entire Max plugin
* Rename plUnifiedTime::GetCurrentTime to plUnifiedTime::GetCurrent to avoid collisions with the Win32 GetCurrentTime define.
* Don't add precompiled header files to the projects unless precompiled headers are being used

Compile times on my Core 2 Duo (Wolfdale) and VS11:
Before (w/ PCH, w/o Max): 23m 21s
After (w/PCH, w/o Max): 20m 11s
After (w/PCH, w/ Max): 20m 38s; (w/o PCH, w/ Max): 25m 5s

We see some small gains from removing unnesecary windows.h includes from the main codebase. The major win here is in compiling the max plugin. Before this is merged, I'd like for someone (like @Mystler) to make sure the max plugin still works properly.