add getPlayMode() for actor and anim interface

[Wizzerinus]

Issue description

As of right now there is not a way to know in which way an Actor is currently playing an animation (playing, looping, etc.) This PR fixes it.

Solution description

I've published a getter for play_mode in AnimInterface and exposed a helper on Actor. since getAnimControls() didn't used to return controls that are not playing anything, it made the helper not work in Pose mode, so I added an optional argument to control that.

Checklist

I have done my best to ensure that…

• …I have familiarized myself with the CONTRIBUTING.md file
• …this change follows the coding style and design patterns of the codebase
• …I own the intellectual property rights to this code
• …the intent of this change is clearly explained
• …existing uses of the Panda3D API are not broken
• …the changed code is adequately covered by the test suite, where possible.

[rdb]

LGTM, thanks