Vastly improved central image loading routine

1) All formats now use a cascading load system (FreeImage -> GDI+ ->
LoadPicture).  This should maximize the number of images PhotoDemon can
load.
2) Better error checking for invalid filetypes via LoadPicture
3) All errors are now suspended until the full list of images has been
loaded.  Then, the user is given a single error dialog if problems
occurred.  This means massive lists of images can be loaded without any
interaction from the user.