Move the pidgin_dnd_* api to its own file.

This is currently only used by the buddy list, but with it being in gtkutils.h,
it causes all sorts of errors in the GTK4 branch because GtkSelectionData has
been removed. Therefore, this change is just about limiting the side effects
on the GTK4 branch.

Testing Done:
compiled and ran, didn't test dnd, but should be fine as I didn't actually change any code.

Reviewed at https://reviews.imfreedom.org/r/1299/