Replace aria-hidden toggling with the aria-modal attribute

[KittyGiraudel]

Demo: https://codesandbox.io/s/wonderful-fermat-bmovn?file=/src/a11y-dialog.js
Docs: #164
Requirements: #158
Changes:

• The container property is now this.$el
• The dialog property no longer exists
• The role attribute is added by the script if not present
• The aria-modal attribute is added by the script
• The open attribute is no longer manipulated
• The concept of targets no longer exists

[KittyGiraudel]

Let’s push that for v7, and remove the concept of “targets“ entirely.