Add controls to set the clipping values

Adds controls in the KStars settings menu, under FITS, to set the values used to mark pixels as red/clipped when the clipping button is selected. Rudimentary implementation--doesn't automatically redraw when the clipping value is changed, you need to toggle the clipping button twice to do that. Also, slow implementation (works pixel-by-pixel) which is OK when there are not too many clipped pixels, e.g. 1000s, but if you chose a very low clip value so that there were millions of clipped pixels it might freeze KStars. Therefore, I implemented a 3s timeout--clipping fails if it takes longer than that.