[THEME] Make the module public.

 * Added new public header mctrl/theme.h
 * Reworked src/theme.c
 * Updated all callsites (most controls).
[THEME] Be more verbose about current theming usage.
[THEME] Fix typos.
[THEME] Do not attempt to GetProcAddress of SetWindowThemeNonClientAttributes().

It is not exported from UXTHEME.DLL, it is only inlined function in <uxtheme.h>.