Add a global Vue.depTarget api support

[yuchonghua]

Add a global Vue.depTarget api
Enable vuex to use the same domain name across windows
Cross-window includes window.open and iframe

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:Add a global Vue.depTarget api support

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
Resolved demo:vuex-open-window.zip

unzip vuex-open-window.zip
cd vuex-open-window
npm i
npm run dev
open "http://127.0.0.1:8080"

more-nuxt-issues-unsolved

[yyx990803]

信息太简略了，请用中文完整地解释这个 PR 的需求场景。

[yyx990803]

用例我明白了，有时间我会看的，不要老是 at 我。

[yuchonghua]

🙂