Decided not to use SendMessage() in the Windows uitask() as we're already waiting for an event return anyway. Decided not to move the message-only window to an init file, as the message-only window is for uitask().
Added GetWindowLongPtr()/SetWindowLongPtr() generation to the windowsconstgen tool and the pregenerated files. This will be needed for when we get rid of creating a new window class for each Window/Area. Also did some minor cleanup to the windowsconstgen tool.
Normalized DefWindowProc() handling across the Windows files; the syscall.LazyProc now has the normalized name _defWindowProc and defWindowProc() is a convenience function that calls _defWindowProc properly. This will also be important for the switch to a single window class per Window/Area.
Moved the standard window class (for Window) on Windows to get the sysData pointer from CreateWindowEx() and store it in the window memory instead of being given it via a closure. This will lead to having only one window class for all Windows, which will come next. Also fixed an error in windowsconstgen's output related to GetWindowLongPtr()/SetWindowLongPtr().
Some quick formatting changes to stdwndclass_windows.go.
Converted the standard Windows window class (for Window) to be a single class, rather than having a new one for each Window. Now for Area.
Changed Areas on Windows to store their sysData inside the window memory instead of being given it via a closure. Actually having only one window class for all Areas comes next.
Changed Areas on WIndows so that they all use the same window class, rather than having one per Area.
Cleaned up the classType struct and sysData.make() in sysdata_windows.go to remove unused fields and the window class registration code now that we no longer register window classes on the fly for each new Window/Area.
Changed storeSysData() on Windows to write the HWND to the sysData structure there. This will be important for removing some of the TODOs from areaWndProc().
Removed TODOs about the sysData not being initialized at message time in area_windows.go; we have now solved that problem.
Removed the future plan about not generating window classes on Windows for each new Window/Area now that we did that.