audio: add device selection & listing with --audio-device

Not sure how good of an idea this is.

This commit doesn't add support for this to any AO yet; the AO
implementations will follow later.
ao_alsa: implement device listing & selection

Unfortunately, ALSA is particularly bad with this, because mpv has to
add all sorts of magic crap to the device name to make things work. The
device selection overrides this, so explicitly selecting devices will
most likely break your audio. This has yet to be solved.
client API: add an explanatory comment

So someone reading this at least has a chance to find out what this is
needed for.
x11: fix inverted condition

Worryingly wrong. Fixes #1162.

Also fix another issue (window title was set anyway), which was why I
didn't notice this and testing it seemed to be fine.
manpage: fix --audio-pitch-correction description

Add closing ">" and specify what is the default value.
libmpv/cocoa: make global events work and get rid of is_cplayer

After @frau's split of macosx_events from macosx_application, `is_cplayer' is
not needed anymore. At the moment only global events such as Media Keys and
Apple Remote work, because the VO-level ones were hardcoded to be disabled.
(that will be fix in a later commit ).
cocoa: fix 'uninitalized' access

I think this doesn't make a difference, since in Objective-C nil responds as
a NullObject, but better not depend on this crappy language feature.
libmpv/cocoa: don't start the event monitor

The event monitor is used to get keyboard events when there is no window, but
since it is a global monitor to the current process, we don't want it in a
library setting.
fix -Wduplicate-decl-specifier warnings with clang
cocoa: post keydown and keyup events without event monitor

Our code worked under the assumption that the event monitor is always active
and we did remove the keydown and keyup overrides from our cocoa view.
fix -Wvisibility warnings with clang

Now everything compiles with no warnings! yay!