* app/image_map.[ch]: new function image_map_clear that removes
	the preview without freeing the image_map.

	* app/brightness_contrast.c
	* app/color_balance.c
	* app/curves.c
	* app/hue_saturation.c
	* app/levels.c
	* app/posterize.c
	* app/threshold.c: Add a call to image_map_clear in the
	preview toggle button callback.  This makes the preview toggle
	button behave as expected.

	* app/histogram_tool: remove an unnecessary include.