Now each platform-dependent module declares its interface in
platform/platform_module.h file and after that includes platform-
specific headers like platform/win/module_win.h with implementation.
Also removed the legacy WinRT platform-dependent implementations.
Also fixed drag-n-drop images from Firefox in Windows.
Also some additional colors added to themes palette:
HistoryToDown button now has separate colors.
Windows title bar and buttons have separate active/inactive colors.