fix(datepicker): simplify focus trap with observables

[maxokorokov]

Refactor existing implementation of the focus trap: handle only inside clicks and tab navigation. It doesn't steal outside focus anymore

Fixes #2406
Fixes #2390
Fixes #2372 (one more time)

[benouat]

For now this PR looks good to me.

Nonetheless, as soon as we have Protractor tests on the project, we should add a lot more test for both focustrap and any widget using it.

[pkozlowski-opensource]

@maxokorokov @benouat I've rebased this locally and can see 2 tests failing:

• ngbFocusTrap should re-focus previously focused element inside the focus trap element FAILED
• NgbInputDatepicker focus should re-focus elements inside the datepicker FAILED

[pkozlowski-opensource]

I would vote for dropping unit tests that try to test focus() - this is just too fragile. IMO cost to benefit is just not there.

[maxokorokov]

Rebased PR

@pkozlowski-opensource this is strange, as I haven't seen it locally and sauce labs didn't have any issues with these tests. Something went wrong during rebase, maybe?

I would prefer to keep at least these 2 tests before protractor is there. Will remove them if they're really unstable.