Binding and API fixes

[Marioalexsan]

Fixed binding signatures (which were originally broken by yours truly):

CSFML_WINDOW_API sfBool sfContext_isExtensionAvailable(const char* name);
CSFML_WINDOW_API GlFunctionPointer sfContext_getFunction(const char* name);

Removed old bindings that don't exist anymore in CSFML:

private static extern bool sfRenderTexture_saveGLStates(IntPtr CPointer);
private static extern bool sfRenderTexture_restoreGLStates(IntPtr CPointer);
private static extern bool sfRenderWindow_saveGLStates(IntPtr CPointer);
private static extern bool sfRenderWindow_restoreGLStates(IntPtr CPointer);
private static extern uint sfRenderWindow_getFrameTime(IntPtr CPointer);
private static extern uint sfWindow_getFrameTime(IntPtr CPointer);
private static extern IntPtr sfShape_copy(IntPtr Shape);
private static extern FloatRect sfText_getRect(IntPtr CPointer);
private static extern FloatRect sfTexture_getTexCoords(IntPtr texture, IntRect rectangle);

Fixed missing userdata pointers in SoundRecorder callbacks.

Moved sfMouse_getPosition, sfMouse_setPosition, and sfTouch_getPosition back to Window, since those functions operate on sfWindow structs and not sfWindowBase. WindowBase will need its own CSFML functions that are not available at the moment, which is why I used NotImplementedException for the time being.

Also changed Mouse and Touch functions to use WindowBase instead of Window.

[eXpl0it3r]

Took a while, but finally got around to it.

For SFML.Net 2.6.0, I guess we still have to release CSFML 2.6.1, so we can implement the WindowBase functions.