Do not keep HINSTANCE variables around

Do not keep HINSTANCE variables around

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