Add Win 7 version check

Needed for some (hopefully forthcoming) Win7-specific features.
Rewrite XP menu subclasser to be IDE-safe
Enable dynamic Recent File icons on XP

Thanks to the IDE-safe subclass rewrite, it is now possible to do this!
XP is limited to 16x16 as I do not have a good way to separately
subclass just the recent files menu, but hey - it's better than
nothing...
Fix histogram initialization bug

With thanks to RR for reporting
Further improve XP menu icon appearance

Shortcut captions are back for the MRU
Potentially fix many window manager issues

Fixes #100.
Fixes #101.
Hopefully fixes some stuff described #98.

Changing the window manager is fraught with peril, as the matrix of PD
interface options + OS versions + OS visual themes introduces a lot of
uncertainty.  That said, I think this rather heavy-duty commit improves
window behavior for all PD interfaces and OS variants, but I will feel
better after others have a chance to test.

In this new strategy, toolbars and image windows have their parentage
dynamically reassigned depending on their float status.  This involves
fiddling with window styles and SetParent, as well as a rather large
overhaul of the way individual windows are synced with the main PD form.

The advantage of this system is that children windows do not "steal"
focus from their parent, so in the default tabbed interface, you can
interact with both toolbars and images without greying out the main
menu.

One disadvantage (maybe?) of this system is that child windows no longer
appear separately in the taskbar while in docked mode, but upon further
reconsideration, that is probably the right way to handle it.  In float
mode, image windows will still individually appear in the task bar.

For reasons I do not understand, this change fixed a number of
unpredictable errors, like Win 7 taskbar thumbnails not being updated
correctly.  I suppose I am happy about that...?