Remove Window::getCursorPosition()

Wasn't used and belongs more into the input backend anyway.
Remove InputBackend::getKeyAsText()

Implementation was not backend-specific.
SDL2InputBackend: Remove redundant NULL check
SDL*InputBackend: Don't enable joystick events

We don't use them anyway!
SDL*InputBackend: Enable all required events

Don't assume some events are already enabled by the window.