Made Windows container transparent. Transparency not completely functioning yet...
More TODOs.
Split out common container window procedure code in Windows to its own function.
Moved code to handle window user data in the Windows backend to a single function.
Fixed some small issues in the C code of the Windows port.
Made Labels on Windows transparent.
Resolved Alt key woes on GTK+.
Removed -[NSApp activateIgnoringOtherApps:]; see http://stackoverflow.com/a/25318870/3408572. I don't know how much of a burden this will be in practice.
More future plans.