Add HTIMER type and change dw_timer_() functions to use it. On existing platforms, HTIMER will be int for now allowing API backward compatibility. On new platforms, iOS and Android, these will be pointers to the timer objects. This allows simplifying the code paths on those platforms. May change it on other platforms such as Mac in a future major version.

# Set these here to change the version for all platforms
DW_MAJOR_VERSION=3
DW_MINOR_VERSION=2