CComponents: rework kill()

There was some problems with restore, hide, clean and save background of item parts,
seen in detailsline. kill() worked not really clean.
kill() works now as virtual member and uses paintBackgroundBoxRel().

In consequence of which:
-unnecessary members in subclasses removed.
-add some comments

src/gui/components/cc_item.cpp

@@ -129,28 +129,6 @@ void CComponentsItem::hide(bool no_restore)
 	hideCCItem(no_restore);
 }
 
-
-//hide rendered objects
-void CComponentsItem::kill()
-{
-	//save current colors
-	fb_pixel_t c_tmp1, c_tmp2, c_tmp3;
-	c_tmp1 = col_body;
-	c_tmp2 = col_shadow;
-	c_tmp3 = col_frame;
-
-	//set background color
-	col_body = col_frame = col_shadow = COL_BACKGROUND;
-
-	//paint with background and restore last used colors
-	paint(CC_SAVE_SCREEN_NO);
-	col_body = c_tmp1;
-	col_shadow = c_tmp2;
-	col_frame = c_tmp3;
-	firstPaint = true;
-	is_painted = false;
-}
-
 //synchronize colors for forms
 //This is usefull if the system colors are changed during runtime
 //so you can ensure correct applied system colors in relevant objects with unchanged instances.