Add 4 new API methods

[AxMannessiez]

• isContentLoaded(): Returns true if a content is loaded.
• isRebuffering(): Returns true if the player is rebuffering.
• isPaused(): Returns true if the content is not currently playing.
• getLastStoredContentPosition(): Get the last stored content position, in seconds.

[peaBerberian]

Nice, we now only need to add its documentation in doc/api and doc/reference (I may also do it when I have the time) and maybe to add some simple tests in tests/integration as most should be pretty straightforward to test.