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.