Send mouseleft event when the mouse leaves the window (#928)

* Send `mouseleft` event when the mouse leaves the window

* Call `View:on_mouse_left` when the mouse leaves the `View`

Previously `View:on_mouse_left` was called only when the mouse left the
window, and it was called on every visible `View`.

Now it gets also called when the mouse "changes" `View`, and only the
last `View` the mouse was on will receive the event.

src/api/system.c

@@ -161,6 +161,9 @@ top:
       } else if (e.window.event == SDL_WINDOWEVENT_RESTORED) {
         lua_pushstring(L, "restored");
         return 1;
+      } else if (e.window.event == SDL_WINDOWEVENT_LEAVE) {
+        lua_pushstring(L, "mouseleft");
+        return 1;
       }
       if (e.window.event == SDL_WINDOWEVENT_FOCUS_LOST) {
         lua_pushstring(L, "focuslost");