Expose more stuff for automation

New *aegisub.**** function:

* **active_frame()** - returns active video frame or nil.

The next *aegisub.**** functions load/close corresponding file returning true on success or nil otherwise. 
* **load_keyframes**(path), **close_keyframes**()
* **load_timecodes**(path), **close_timecodes**()
* **load_video**(path), **close_video**()
* **load_audio**(path), **close_audio**()