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/ui.c

@@ -23,7 +23,7 @@
 #include "nvim/normal.h"
 #include "nvim/option.h"
 #include "nvim/os_unix.h"
-#include "nvim/os/event.h"
+#include "nvim/event/loop.h"
 #include "nvim/os/time.h"
 #include "nvim/os/input.h"
 #include "nvim/os/signal.h"
@@ -216,7 +216,7 @@ void ui_detach(UI *ui)
 
   ui_count--;
   // schedule a refresh
-  event_push((Event) { .handler = refresh }, false);
+  loop_push_event(&loop, (Event) { .handler = refresh }, false);
 }
 
 void ui_clear(void)