Add clanUI to Linux build + fixes to compile it.
Shorten .gitignore
Break X11 transient windows to remove #include <X11/Xlib.h> in API.

`Xlib.h` has typedefs using the same names on ClanLib (like `clan::Font` and
X11's `Font`) which causes ambiguity problems when the `clan` namespace is
imported into the global namespace.

There is no API to generically move WM handles, but it seems that it is only
needed when creating multiple WM windows linked to the same top-level parent
window on X11, thus we are disabling it as a temporary workaround.

The Windows build should not affected by this change.
Added DisplayWindowHandle + implementation for X11

This class is used to contain the display manager's window
handle so that ClanLib can obtain it from a `DisplayWindow`
instance. It is implemented as a pointer to an undefined
class declaration on the API which is then defined inside
the whatever windowing system that is compiled with the
library.
Win32 DisplayWindowHandle implementation
Merge pull request #43 from keigen-shu/master

Remove #include <X11/Xlib.h> from API headers + Linux fixes