Some CSFML 2.6.x API Discrepancies

[Marioalexsan]

I've noticed these while trying to update SFML.Net to 2.6.0.

• Mouse is missing position getters / setters for WindowBase (i.e. sfMouse_getPositionWindowBase and sfMouse_setPositionWindowBase)
• Touch is missing position getter for WindowBase (i.e. sfTouch_getPositionWindowBase)
• While it might not be very useful, createVulkanSurface overloads for derived windows are missing (i.e. sfRenderWindow_createVulkanSurface and sfWindow_createVulkanSurface)
• sfKeyboard_getDescription returns an owning const char* that must be freed using C's free, however CSFML doesn't expose something like sfFree / sfMalloc that can be easily called from the C# side, which causes issues with memory leaks

[ChrisThrasher]

Thanks for catching these oversights! Looks like a lot of them are due to me not realizing the true extent to which we'd have to change things while adding sf::WindowBase support. We'll get around to fixing these things. Expect them to ship in 2.6.1. Can't provide any estimated timeline yet for when that will ship.

[Marioalexsan]

Closing since it was fixed by #236.