Now, we have two classes (instantiated from the xib file for each window)
with controls bar stuff:
- VLCControlsBarCommon holds all code common for main and detached window
- VLCMainWindowControlsBar adds code specific for the main window bar

With that, we can avoid all these redundant code for o_detached_*, furthermore
this decouples all detached window control bar stuff from MainWindow.m.

The objects can be accessed through the controlsBar method.