🐛 Fix medium-zoom not updating on component update

[dcastil]

Summary

medium-zoom doesn't get attached to newly loaded images when the user navigates to other routes within a vuepress project. The problem is that medium-zoom gets attached to currently available images when the LayoutDistributor is mounted, but not when it's updated (e.g. when the user navigates to a new route).

My fix detaches all images from medium-zoom when the LayoutDistributor updates (as images could have been removed from the DOM) and attaches all current images in the DOM which match the provided selector.

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

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

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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:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
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• IE

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

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

I wanted to upgrade to medium-zoom v1.0.2 but the vuepress build command threw an error that the window object isn't defined. I don't understand why that happens as the window object is also used in v0.4.0. The error appears when medium-zoom tries to access window.Promise.

[ulivz]

LGTM