This requires refactoring NewCloseButton() in order to extract
CreateCloseButton() from it, as XRC relies on being able to use two-step
creation which was previously impossible for this kind of buttons.
CreateCloseButton() is rather unusual, as it has to be declared in the
derived, platform-specific class, in order to be able to call its
Create(), but is defined only once in common, platform-independent,
code. However the only alternative seems to be to have a static
function, e.g. InitCloseButton(), which wouldn't be very pretty neither.
Closes https://github.com/wxWidgets/wxWidgets/pull/2118
712c2d4004