Improve wxCollapsiblePane appearance under OS X.

Improve wxCollapsiblePane appearance under OS X.

By default, don't use border in wxDisclosureTriangle because the native
applications do not. However do honour wxBORDER_XXX style if specified in case
this becomes useful in the future (this required no effort as the code was
already there for bitmap buttons and just had to be extracted into a reusable
function).

Don't expand the disclosure triangle in wxCollapsiblePane sizer, as this
resulted in the text being centered instead of remaining left-aligned.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62431 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775