LinuxWindow updated to use new WindowImpl design.
Other features that were implemented in WindowsWindow were added to LinuxWindow.
However, not all features are supported yet (e.g. popup / "borderless" windows, control over whether or not user can resize window, etc)

Added LinuxThreadLocal, the implementation of ThreadLocal for Linux (using pthread's faculties).

Updated makefiles, added "--Wl,no-as-needed" option as a work-around to a bug preventing use of std::thread in G++ 4.8.1.

Fixed a compilation issue in window/WindowEvent.hpp that occured when compiling with g++.

Minor cleanups in WindowsWindow code