Extend layer_t with cornerRadius & borderColor

This allows drawing rounded layers with border in resolution independent
manner. Also it reduces number of layers needed to certain UI elements.
Define color names for OakTabBarView

This lets layout file refer to color names instead hardcoding color hex values.

Also colors referenced by name can return different values depending on OSX
version, which will in future solve problems with OSX 10.10 Yosemite L&F.
Reworked OakTabBarView layout plist

Now it supports properly Retina using Cocoa rounded corner rendering instead
pixel patches. All colors are now references by name.
Tune tab label inset shadow

Now it matches more way other OSX elements are inset both on Retina and
non-Retina displays.