fix: move auto installation code into the store constructor

[ktsn]

While auto-calling Vue.use(Vuex) is convenient for users who use Vue/Vuex via <script>, there are a case that users want to avoid the behavior when window.Vue is exists (See #731).

Currently, Vuex is immediately installed into window.Vue when Vuex is imported. So there are no way to install Vuex into imported Vue.

To solve this, I'd move it in Vuex.Store's constructor. Then the auto-installation is delayed until users create a store and the users can install Vuex into their own Vue constructor before doing it.

I'm not sure how to write test code for this but it looks like working well.

fix #731

[yyx990803]

Since the constructor can be called multiple times, we need to make sure install is called only once.

[ktsn]

I thought existence check of inner Vue is enough because when install is called, the inner Vue will be assigned. Then install will be skipped on the next time.
Am I missing any other cases?

[yyx990803]

Ah right, nevermind! LGTM

[ianrussel]

I have same problem
[vuex] already installed. Vue.use(Vuex) should be called only once.

When using window.Vue

I fixed it by
bootstrap.js

 window.Vue = require('vue');
  window.Vuex = require('vuex');
  Vue.use(Vuex); 

main.js

      import store from './store/store.js';
      import Papp from './Papp';

     import VueRouter from  'vue-router';

     const router = new VueRouter({
         //mode: 'history',
     routes: [
      {
         path: '/vue/dashboard',
         component: dashboard
      },
     {
         path: '/vue/dashboard/love',
         component: love,
     }
     ]
    })

     const vm = new Vue({
        el:'#vue',
        router,
       store,
        render: h =>h(Papp),
    });

store.js

         export default new Vuex.Store({
             state: {
                   todos: [],
                   newTodo: '',
             },
           mutations: {
              GET_TODO(state, todo) {
                   state.newTodo = todo
            },
         },
         actions: {
             getTodo({commit}, todo){
                     commit('GET_TODO', todo)
             },
            addTodo({commit}){
               commit('ADD_TODO')
           },
         .................
          },
         getters: {
                  newTodo: state => state.newTodo,
                 todos: state => state.todos.filter((todo) => {return   !todo.completed}),
                 completedTodos: state => state.todos.filter((todo) => {return todo.completed})
                  }
       })