Explicitly require Python version 3 for plugin scripts and automatically choose suitable binary.

Python binary could be named as "python" or "python3" - Synfig checks the version at runtime and chooses suitable binary.
When set a Cairo surface it is needed to map it i order to be used by the importer
Fig bug #418 - Image isn't displayed if it's grouped (Cairo)
Layer Paste Canvas now passes the render method to its canvas.
Fix bug #422: Crash: map_cairo_image(): Assertion failed 

There weren't any checking of the status of the cairo_surface_t when reading the image from file. This commit adds it and fixes the bug.
Bug report #420 - The Add keyframe button shouldn't be active if there is not timeline