Fix the textbox layout behavior, add DXSDKDIR to premake and restore default skin

This commit fix the centering issue of the textbox (broken recently due to selection behavior)
The selection behavior of text box was also fixed to work the way you would expect it to work, (cursor stay at the left or right of an approximate character size instead of jumping to the middle suddenly) 
(I may implement the Multiline textbox selection later on, but this is a bit more tricky)

The DefaultSkin.png was modified to restore the default skin of some controls, mostly textBox. Was it removed on purpose ? (This make the samples look broken :s)

I added the DirectX SDK include and bin dirs to the premake file (only for DX renderers and sample) so that it compile with msvc2010, adding the SDK as a global include in VS is not recommended.