Freetype based text rendering not supported on OSX

Kerning and hinting is handled differently on different platforms. Therefore text runs differently, which causes layout and style issues in multi-platform applications. A common solution to this problem is to use the same text engine on each platform, Freetype for instance. Currently the OSX platform plugin only supports native text rendering. Therefore the possibility to chose Freetype for text rendering on OSX should be added. For instance by passing "-platform cocoa:fontengine=freetype" on command line, just like the Windows platform plugin demonstrates.