Minimum horizontal area size should be scaled by UI resolution.

We intend to keep editor sizes larger than minimum sizes. For the vertical minimum we never screw this up because we generally call ED_area_headersize() which is scaled.

However, for horizontal minimum we tend to use the define AREAMINX (currently 32), and which must be scaled by the UI resolution. And we sometimes forget to do so. This patch fixes that.

The problems we get with this issue occur when we use a combination of AREAMINX that is not scaled against values that are. We allow splitting on areas that are too small, joining with participants that are smaller than allowed. Here is the Properties editor resizing to 1/3 the intended width because the UI scale is set to 3X:

While looking into this, I saw that space_buttons has its navigation bar prefsizex set to AREAMINX - 3 with a comment of XXX Works and looks best, should we update AREAMINX accordingly?

The answer is yes. It looks really nice to have AREAMINX set to the same as the width of Properties Nav bar, and that looks best at 29. So this patch changes that too.