Fixed SoundEffect's copy assignment operator.
Delay generation of BitmapText destructor to cpp file.
Test branch.
Added Pimpl class to replace unique_ptr pimpls.
Merge branch 'pimpl-test' of github.com:Gazok/Tank into pimpl-test

Conflicts:
	Tank/Audio/Music.cpp
	Tank/Audio/Music.hpp
	Tank/Utility/Pimpl.hpp
	Tank/Utility/Pimpl_impl.hpp
Added const members to Pimpl.
Updated BitmapText to use the new pimpl class.
Updated FrameList to use new Pimpl class.
Updated text to use new Pimpl class.
Removed unnecessary include from observing_ptr.