Replaced Xlib by XCB implementation.

* Added FindXCB.cmake script
* Added AutoPointer wrapper for automatically free'ing pointers
* Huge commit: Ported linux implementation of sfml-window to xcb
* Xcb is now used for window creation, event loop etc
* As GLX is linked to Xlib, that part of the implementation
  still uses Xlib.
* Also, some keyboard related (such as XLookupString) stuff
  is still Xlib, as xcb does not have it (yet?).
* Replaced some enums with the xcb equivalents
Adjusted and fixed XCB patch.

* Adjusted xcb_icccm calls (for recent XCB versions).
* Fixed wrong parameter order in xcb_icccm_set_wm_protocols call.
* Fixed XCB_BUTTON_RELEASE spawning a MouseButtonPressed event.
* Moved files from obsolete Linux/ to Unix/ directory.
* Added m_useSizeHints fix.
* setTitle() converts to UTF-8 before passing to XCB -> Unicode window title
  support.
* Added XCB-util dependency.
* Replaced XSelectInput. Obtaining XCB connection when taking window handle.
* Adjusted X11 example for XCB.
* Removed AutoPointer, replaced by direct XCB and free() calls.