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This PR attempts to solve the problem described in #2169,

Motivation

In our project we have popovers with information that users might like to copy, and also popovers with links for users to click. I consider these scenarios fairly basic and common, but ngbPopover doesn't really support them because it closes as soon as the user moves the mouse out of the element the popover is bound to (I will call it the target element from here on).

Here are some "solutions" we considered:

• Force our users to make a click to open the popup
• Confugure [closeDelay] large enough for users to be able to click a link before the popup closes
• Use code snippets from #2169.

None of these are acceptable.

Proposal

The basic idea is to listen to trigger events both on the target element AND the popover element. To illustrate, in case of triggers="mouseenter:mouseleave" the popover will close on mouseleave UNLESS mouseenter was triggered when the user moved the cursor from the target element to the popover.

Of course, the cursor isn't moved instantaneously, so some delay is required. You can find it in the new NgbPopoverConfig.mouseleaveCloseDelay field and NgbPopover.mouseleaveCloseDelay input. I chose 100 ms as the default, but you are welcome to suggest any other positive number. I will tell you more, there is a similar field and a similar input for focusout as well.

You might argue that this approach is wrong and these events shouldn't be treated as special citizens. Maybe they shouldn't, but although I am open to suggestions regarding design and refactoring I would prefer to keep the code dumb and not over-complicate things until the basic idea is approved and my approach reviewed and tested. (Also I must note here that events will need to be handled differently anyway because they require different delays: mouse movement does not happen instantly while clicks and focus changes do.)

Among the enhancements themselves this PR includes:

1. A couple of new unit tests which appropriately fail without the enhancements
2. A couple of new buttons on the popover demo page so that the updated behavior can be seen in action.
3. Similar enhancements, unit tests and demos for ngbTooltip

There are some technical details I would like to point out:

1. In order to detect focus events, a popover needs to be focusable (have tabIndex)
2. "Click" triggers are excluded from the general rule: in contrast to other trigger events, clicks are not listened to on the popover element. This is because clicks outside the target element are already nicely handled by autoClose.

Update: fixed some typos.