[GRID,MDITAB,TREELIST] Rename XXX_mcchanged() to XXX_theme_changed().

They were accidentally renamed to XXX_mcchanged() with "Replace all"
when reorganizing the theme module.
[TREELIST] Support for item icons.

 * Added messages MC_TLM_SETIMAGELIST and MC_TLMGETIMAGELIST.
 * Updated treelist_paint() to paint item image if available.
 * Updated treelist_hit_test() to support MT_TLHT_ONITEMICON.
[TREELIST] Update examples ex_treelist2 to use item icons.