Multi-point Touch Support

[jpsamaroo]

Currently uGUI only has support for a single touch point, however certain dev boards with capacitive touch sensors (the STM32F7-Discovery, for example) have up to 5 or more points that may be activated simultaneously. While the simple solution is to just pass all 5 (or more/less) in one at a time, I think there is room for growth of uGUI in such a way that these multi-touch events could be used to add additional functionality. One such example is rotating a window, where two touch points could be used to adjust the rendering of the window to make it appear rotated, leading to a potentially improved experience and greater usefulness. Not strictly necessary, but something to consider I think.