Fix T41708, active palette indicator not refreshing when clicking

Issue, after a lot of blood sweat and tears, was found in
ui_but_update_from_old_block, where we restore a button to its old
values when possible. The problem here is that a1 and a2 are not really
meant to store temporary variables, because they tend to get overriden
and palette selection is one of those temporary states.

Instead, we now store the position of each button in the palette in a2
and pointer to the palette in the customdata pointer of each button and
use that to test if it's active. The positions won't change when
clicking so we are guaranteed that the old button won't override the new
one with garbage.

It's still hacky but it is better than testing button types when copying
old values.

source/blender/editors/interface/interface_handlers.c

@@ -4318,9 +4318,7 @@ static int ui_do_but_COLOR(bContext *C, uiBut *but, uiHandleButtonData *data, co
 		else if ((int)(but->a1) == UI_PALETTE_COLOR &&
 		         event->type == DELKEY && event->val == KM_PRESS)
 		{
-			Scene *scene = CTX_data_scene(C);
-			Paint *paint = BKE_paint_get_active(scene);
-			Palette *palette = BKE_paint_palette(paint);
+			Palette *palette = but->rnapoin.id.data;
 			PaletteColor *color = but->rnapoin.data;
 
 			BKE_palette_color_remove(palette, color);