GUI: Various GUI Improvements

These commits include various improvements to the ScummVM GUI:

Soft shadows on rounded squares and tabs
Gradients on borders of rounded squares
Partial text rendering (in the about-dialog as well as in text-input fields)

I've tried matching these to be as close to the mockups that are available. I've also squashed these individual features into individual commits to make it easier to test and read through the code. The shadows also include a special case to make them less soft when using a 8-bit overlay for the GUI, since we will have strange colors on the shadow otherwise. I've attached two screenshots where the GUI is rendered in 32-bits:

![2013-07-29 20-29-19](https://f.cloud.github.com/assets/4008711/873671/c6507f6a-f87c-11e2-8b99-b10dabca76af.png)

![2013-07-29 20-29-05](https://f.cloud.github.com/assets/4008711/873676/effef300-f87c-11e2-869b-b7216ba13e22.png)