Clear out the content container rather than deleting it

[tristen]

Launch Checklist

This PR changes behavior in popup.js. Currently, when setDOMContent is called, it destroys the content container, creates a new one, and populates. This is undesirable under situations where you don't want to introduce a CSS file as a dependency to a plugin and wish to modify the content container using JavaScript.

Right now I'm calling this every time I want to add new data (which is happening on mouseover when queryRenderedFeatures returns a response):

const selector = popup.getElement().querySelector('.mapboxgl-popup-content');
selector.style.padding = '0'

Without modifications	With modifications

I would prefer to modify the DOM once and not have this be repeatedly called.

I'm a little nervous this constitutes a breaking change even though it's minor. Curious what y'all thoughts are.

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes
• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Clear out the popup content container instead of deleting it</changelog>

[ryanhamley]

Looks good!