OSX: Implement TaskbarManager for Mac OS X

This implements count badge, progress bar, icon overlay and error notification overlay. Recent items is not implemented.

It uses the NSDockTile API which is available since OS X 10.5. Currently you have to explicitly enable it when running configure (i.e. it is disabled by default). It would be possible to modify the code in such a way that when compiled with task bar enabled it would still work (without any of the taskbar features) on older systems (using NSClassForString). But I am wondering if this is worth it.

Here are a few pictures (the game icon are from the icon thread on the forum):
BASS icon overlay
![scummvm_dock_bass](https://f.cloud.github.com/assets/552105/2005010/51e863c0-86cc-11e3-93a8-915666c8cb1c.png)

Drascula icon overlay
![scummvm_dock_drascula](https://f.cloud.github.com/assets/552105/2005012/64819fc4-86cc-11e3-9693-f3d44cf53890.png)

Error overlay
![scummvm_dock_error](https://f.cloud.github.com/assets/552105/2005013/7130a99a-86cc-11e3-9123-f2d1c1271f39.png)

Mass Add
![scummvm_dock_massadd](https://f.cloud.github.com/assets/552105/2005015/7738dccc-86cc-11e3-8b50-8824cb64bf90.png)