Improved mouse buttons handling

Following @NathanSweet commit 00526c7ee0976a66322b30d8f5fe73bbb557cc63 (regarding issue #2391), I made the same changes to the JGLFW and Android backend to ignore any unknown button.

I also added actual support for mouse buttons 4 & 5 on all 3 backends. (LWJGL, JGLFW, Android)

I didn't mess with GWT since the platform doesn't actually support anything beyond the first 3 mouse buttons anyway. (That and the fact that the DEV Plugin doesn't work on 64-bit Chrome and on any recent version of Firefox so I couldn't test any change I'd make.)
Yup, that's a line of debug code that I forgot to remove.