libwine: Fix the wctomb validity check for codepages where the default chars don't convert to each other.

user32: When setting a menu item, handle MIIM_TYPE flag by setting approriate MIIM_BITMAP, MIIM_FTYPE and MIIM_STRING flags.

user32: Put some repeated code for checking and extending of a MENUITEMINFO structure in a separate function.

Now that we don't depend on activeContext and lastActiveRenderTarget to be
non-NULL this code can go.