Fix bad shift when squeezing with 3D borders disabled

Hi,

When 3D borders are disabled, squeezing a window produces a little shift 
on x by the border width, and the width of the title grows by 2 × the 
border width. Yes, it is a little annoying :)

The fix: 
https://github.com/maxatome/ctwm-mirror/commit/6f0f08ca4aea429424a3b3abf455a84c4022a778

It is weird (but probably historic?) that frame_bw and frame_bw3D fields 
have the same meaning for the user (the border width, but one for flat 
design and the other for 3D), but are not used equally in the code. I 
explain:

  * title_x = frame_bw3D - frame_bw
    ⇒ why not: frame_bw3D + frame_bw?
  * title_width includes 2 * frame_bw but not frame_bw3D

As they are exclusive, I think it should be possible to use them 
equally. And if they are, we can even remove frame_bw3D and only use 
frame_bw in all places where border width matters. What do you think 
about this?

++

Max.