Cocoa: create QCocoaMimeTypes for Cocoa-only mime-types

Prepare to move qmacmime out of the cocoa port and into
QPlatformSupport for sharing with iOS. This patch will
start by moving tiff out, since it depends too much on
old carbon image manipulation code that will not compile
on iOS (depends on ApplicationServices etc).
Cocoa: move qmacmime to platformsupport

Move qmacmime to platformsupport so it can be shared between
cocoa and ios.

Change-Id: If164b926b54240911a66f2a7f809d0066abab182
qmacmime: bugfix unicode::canConvert

The UnicodeText convertor is a bit special since it can convert
to both utf8 and utf16 for text/plain. But since flavorFor("text/plain")
can only return one UTI, it returns utf16. And this means that
canConvert will return false if given the utf8 as argument.
On iOS this is often the only format available on the pasteboard, which
means that pasting text from another app into a Qt app will fail.

This patch will fix the text so that it returns true for both utf8 and
utf16.

Change-Id: Iac3fd8cd235fbad4e27c707c0d56a3c3d12848ee