[DOC] Typo fixes and minor improvements.
[DOC] Add section about DLL dependencies.
Reorganize headers and update docs accordingly.

 * mCtrl/defs.h has been split into mCtrl/_defs.h and mCtrl/_common.h.
 * Headers supposed to be not included directly by apps now begin with underscore.
 * Public headers do not include <commctrl.h> to avoid namespace pollution.