[EXPAND] Check if ->font is NULL before usage.
[MISC] Provide few functions for measuring sizes...

... of some objects:

 * mc_string_size()
 * mc_string_width()
 * mc_string_height()
 * mc_icon_width()
 * mc_icon_height()
[CHART] Use mc_string_width() instead own implementation of the same.
[MDITAB] Add support for dynamic item width.

If mditab->def_width is zero, the item width is determined dynamically
according to its label and icon size. Also, it is now default setting
when the control is created.

Also updated example to not set the desired item widths explicitly to
use it.