Fix bezel used for bitmap buttons in wxOSX/Cocoa.

Fix bezel used for bitmap buttons in wxOSX/Cocoa.

Don't use NSRoundedBezelStyle for bitmap buttons as this bezel has fixed
height. Instead, use NSRegularSquareBezelStyle which can used with buttons of
any size and is the correct bezel to use for the buttons mostly identified by
their icon according to Apple docs.

Notice that we still use the standard bezel for the "small" (where "small" is
arbitrarily defined by the hard coded 20 pixels height) icons as those are
usually used in addition to the text and not replacing it and so it makes more
sense to use the same bezel as for the normal buttons for them.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73056 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775