Also, adding support for devices which report fractional scrolling
amounts. E.g. a mouse with a continuous mouse wheel, or a Mac touch
pad. This is currently only supported in lwjgl3. It can probably be
added to android and maybe to gwt.

InputProcessor scrolled method now receives scroll amount for X and
Y. Changed type to float to support devices which report fractional
scroll amounts. Keeping the original scroll method of InputProcessor
for backward compatibility.  Updated InputEvent in scene2d
accordingly: added scrollAmountX, scrollAmountY attributes and
corresponding setters and getters.

Input event queue can receive float scrolling values.

Updated backends accordingly: lwjgl, lwjgl3, android and gwt.