Fix cursor blinking problem

The problem was that when the editor had no events the cursor was not
blinking because the event loop was blocking on wait_event.

Now we no longer calls wait_event without a timeout if the windows has
the focus. When the window has the focus the timeout is set to 1 / fps
so that the cursor can blinks.

In addition we react to the "focus lost" event to ensure the documents
are redrawn without the cursor.

src/api/system.c

@@ -61,6 +61,10 @@ top:
         lua_pushstring(L, "exposed");
         return 1;
       }
+      if (e.window.event == SDL_WINDOWEVENT_FOCUS_LOST) {
+        lua_pushstring(L, "focuslost");
+        return 1;
+      }
       /* on some systems, when alt-tabbing to the window SDL will queue up
       ** several KEYDOWN events for the `tab` key; we flush all keydown
       ** events on focus so these are discarded */