Improve folder name and start menu name for Qt binaries (Windows and Symbian)

On Windows, if you install both Qt for Windows and Qt for Symbian, you'll end up with some confusion.

The default install directory for both is:
c:\Qt\X.Y.Z
and that of course will lead to conflicts if one of them is not changed.

Also, the Start menu folder is called
Qt by Nokia vX.Y.Z (OpenSource)
with no mention of platform.

I'd suggest that both the default directories and the start menu names are fixed so that they don't clash, and that it's obvious which Qt for which platform we're talking about here.