Add mlt_animation and mlt_property_interpolate().

mlt_property_interpolate() only works on a scalar double property at
this time.
Add support for discrete animation including strings.
Add mlt_property_get_double_pos() and mlt_property_get_int_pos().
Add Catmull-Rom spline smooth animation interpolation.
Add mlt_property_set_double_pos() and mlt_property_set_int_pos().
Interpret % after numeric string.
Add mlt_properties_set/get_int_pos and Properties::set/get_int.
Add mlt_rect and mlt_properties_set/get_rect.
Add mlt_properties_set/get_rect_pos for rect animation.
Rename functions with _pos to anim_.
Add mlt_properties_anim_set/get_double().
Add mlt_properties_anim_set/get() for string.
Add mlt_color and mlt_properties_get_color().
Make animation length optional.

It is only really needed when using negative time values.
With some fixes for parsing negatives in time code/clock values.
Reorder some property anim parameters for consistency.
Update C++ symbol version script for new Properties methods.
Fix missing math.h include when lrint was added.
Add mlt_properties_set_color().
Add mlt_properties_get_animation(); it might come in handy.
Reorder the last commit of mlt_properties_get_animation().