New keyframe types (#957)

* Add more keyframe unit tests

* Create a map of keyframe types and string values

This can be used to easily add more keyframe types

* Add new keyframe types: smooth_natural and smooth_tight

Both new types use centripetal catmull-rom interpolation.
The centripetal method ensures there will be no cusps in the spline.
Additionally, modify the interpolation to avoid overshoot.

These changes ensure that a line will interpolate directly from one
value to another without allowing the interpolated values to reverse
along the way.

The previous "smooth" type is aliased to "smooth_loose" and maintained
for backwards compatibility.

* Target correct MLT version for new functions

* Add multiple easing keyframe types

Easing functions are based on Robert Penner's Easing Functions
http://robertpenner.com/easing/

src/framework/mlt.vers

@@ -681,3 +681,10 @@ MLT_7.18.0 {
   global:
     mlt_audio_free_data;
 } MLT_7.16.0;
+
+MLT_7.22.0 {
+  global:
+    mlt_property_is_color;
+    mlt_property_is_numeric;
+    mlt_property_is_rect;
+} MLT_7.18.0;