Fade dynamics. Add fade as one more dynamics driving factor on GUI. Implement fading dynamics calculation. Let fade work with all driven factors. Add pixel_dist parameter to get_dynamic_x. Adjusted function calls for get_dynamic_x.

        * app/paint/gimppaintoptions.c: Add initialization for fading dynamics. Add fading related variables similar to Random. Adjust get_dynamics_mix function. Add fading calculation in get_dynamics_x functions. Adjust get_dynamic_x functions for fade dynamics.

	* app/paint/gimppaintoptions.h: Add fading_options in GimpDynamicOptions.

	* app/tools/gimppaintoptions-gui.c: Add the function fading_options_gui.

	* app/paint/gimpclone.c
	* app/paint/gimpdodgeburn.c
	* app/paint/gimperaser.c
	* app/paint/gimpheal.c
	* app/paint/gimppaintbrush.c
	* app/paint/gimpsmudge.c
	* app/paint/gimpairbrush.c
	* app/paint/gimpbrushcore.c
	* app/paint/gimpconvolve.c: Adjust function calls. get_dynamic_opacity function calls were adjusted to have one more parameter, paint_core->pixel_dist.

some key words:
get_dynamic_
pixel_dist
paint_core->pixel_dist