audio/out: fix active waiting during pause again

This was fixed in commit 8432eaefa, and commit 39609fc1 of course broke
it again. This was pretty stupid.
player: minor refactoring

Expose the central event handling functions explicitly, so that other
parts of the player can use them.

No functional changes. Preparation for the next commit.
lua: synchronously wait until scripts are loaded

This makes the player wait until each script is loaded. Do this to give
the script a chance to setup all its event handlers. It might also be
useful to allow a script to change options that matter for playback.

While waiting for a script to be loaded, the player actually accepts
input. This is needed because the scripts can execute player commands
anyway while they are being "loaded". The player won't react to most
commands though: it can't quit or navigate the playlist in this state.

For deciding whether a script is finally loaded, we use a cheap hack: if
mpv_wait_event() is called, it's considered loaded. Let's hope this is
good enough. I think it's better than introducing explicit API for this.
Although I'm sure this will turn out as too simplistic some time in the
future, the same would probably happen with a more explicit API.