* app/*.[ch]: Actually use the enum types GimpImageType,
	GimpImageBaseType, LayerModeEffects, PaintApplicationMode,
	BrushApplicationMode, GimpFillType and ConvertPaletteType, instead
	of just int or gint. Hopefully I catched most of the places
	where these should be used.

	Add an enum ConvolutionType, suffix the too general constants
	NORMAL, ABSOLUTE and NEGATIVE with _CONVOL. Use NORMAL_MODE
	instead of NORMAL in some places (this was what was intended). Fix
	some minor gccisms.

	* app/apptypes.h: New file. This file contains the above
	enumeration types, and some opaque struct typedefs. It was
	necessary to collect these in one header that doesn't include
	other headers, because when we started using the above mentioned
	types in the headers, all hell broke loose because of the
	spaghetti-like cross-inclusion mess between headers.

	(An example: Header A includes header B, which includes header C
	which includes A. B uses a type defined in A. This is not defined,
	because A hasn't defined it yet at the point where it includes B,
	and A included from B of course is skipped as we already are
	reading A.)