Use &__ImageBase for the GTK DLL and GetModuleHandle (NULL)
for the application module. Then remove DllMain as it's not
necessary anymore.
References:
[1] Accessing the current module's HINSTANCE from a static library:
https://devblogs.microsoft.com/oldnewthing/20041025-00/?p=37483
Include the appropriate headers as some function prototypes were moved lately.
Also, re-order the include order of the gdk/*private.h headers alphabetically
in the files that were updated.
