Although showPopover continues to toggle the popover (showing if hidden / hiding if shown), hidePopover now will only hide the popover. Furthermore, isPopoverShown has been introduced to provide insight into whether the popover is currently shown or not, and therefore whether it should be hidden or shown.
Also, the showPopover / hidePopover test has been refined to override `trigger` and `hideOnClick` settings to allow for full imperative control of showing / hiding the popover.
* Use QUnit `module` instead of `discourseModule`
* Use QUnit `test` instead of `componentTest`
* Use angle-bracket syntax
* Remove jQuery usage
* Improve assertions (and actually fix some of them)
Note this commit also introduce a new {{d-popover}} component, example usage:
```hbs
{{#d-popover |state|}}
{{d-button label="foo.things" class="d-popover-trigger"}}
<div class="d-popover-content">
Some content
<div>
{{/d-popover}}
```
