x11: Merge Unmanaged into X11Window

Currently, managed and override-redirect windows are split in two types:
X11Window and Unmanaged. While looking at it strictly from type
perspective, this is great. But it creates other problems, e.g. we need
to put shared X11-specific code in the base Window class or mess with
"base" classes.

As an alternative solution, this change merges the Unmanaged class into
the X11Window class and disables some functionality based on the value
of isUnmanaged().

X11Window::manage() is used to create a managed Window. X11Window::track()
is used to create an unmanaged Window.