Patch/retina tabs

This fixes look of tabs for Retina displays.

Previously rounded corners were done by pixel knitting in `TabBar.plist` layout file. Now `layer_t` is enhanced with `borderColor` and `cornerRadius` properties so we can draw rounded corners independently from resolution.

Also all colors in `TabBar.plist` are now referenced by name, instead HEX hardcoded, so we can easily upgrade to *Yosemite* lighter look soon.

### Before

![tabs-active](https://cloud.githubusercontent.com/assets/103067/3292389/d6c43386-f58e-11e3-9f95-dce695569ba4.png)
![tabs-inactive](https://cloud.githubusercontent.com/assets/103067/3292392/dc0524b8-f58e-11e3-8abc-d67be041f0de.png)

### After

![tabs-fixed-active](https://cloud.githubusercontent.com/assets/103067/3292528/4e9a306c-f590-11e3-8c23-2f00d15c1953.png)
![tabs-fixed-inactive](https://cloud.githubusercontent.com/assets/103067/3292501/f3b1ffc2-f58f-11e3-8312-8dc7f84d6920.png)