[MDITAB] Make new layout of items keep scrolling position in allowrd range.
[MDITAB] mditab_layout() updates hot item.
[misc.h] SelectClipRgn() expects device coordinates.
[MDITAB] Add style MC_MTS_ANIMATE to animate some operations.

 * When set, animate scrolling and item layout changes.
 * Update docs.
 * Make example use the style.