TUIImage backed with NSImage

[adib]

Hi,

These changes adds a subclass to TUIImage that is backed by an NSImage instance (not CGImageRef). The benefits are:

• Supports PDF-based images.
• Brings in NSImage's support for multi-resolution image resource files (e.g. retina display @2x files).

Thanks.

[CLAassistant]

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1 out of 4 committers have signed the CLA.

:white_check_mark: adib
:x: joshaber
:x: Justin Spahr-Summers
:x: jspahrsummers

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