event loop: New abstraction layer with refactored time/signal API

- Add event loop abstraction module under src/nvim/event. The
  src/nvim/event/loop module replaces src/nvim/os/event
- Remove direct dependency on libuv signal/timer API and use the new abstraction
  instead.
- Replace all references to uv_default_loop() by &loop.uv, a new global variable
  that wraps libuv main event loop but allows the event loop functions to be
  reused in other contexts.

src/nvim/globals.h

@@ -28,6 +28,7 @@
 #include "nvim/menu.h"
 #include "nvim/syntax_defs.h"
 #include "nvim/types.h"
+#include "nvim/event/loop.h"
 
 /*
  * definition of global variables
@@ -1216,6 +1217,7 @@ EXTERN char *ignoredp;
 
 // If a msgpack-rpc channel should be started over stdin/stdout
 EXTERN bool embedded_mode INIT(= false);
+EXTERN Loop loop;
 
 /// Used to track the status of external functions.
 /// Currently only used for iconv().