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/103067/3292398/e7909510-f58e-11e3-871e-0c5e5ddab5e4.png)
![tabs-fixed-inactive](https://cloud.githubusercontent.com/assets/103067/3292401/eb7b8e64-f58e-11e3-9891-090d4c343271.png)