refactor(ui5-popup): Move more common functionality to Popup.js

[vladitasev]

Currently Dialog.js and Popover.js have a lot in common in the open/openBy and close methods.

The goal of this change is to streamline them, and at the same time make Popup.js much more self-sufficient, so that third parties can create custom popups without understanding our low-level APIs such as popup registries, body scroll blocking, etc...

Popup.js:

• is fully complete and functional on its own - it is enough to inherit it and implement isModal
• has no notion of headerText, header and footer, only a default slot
• the hooks in the template are now called beforeContent and afterContent
• has almost no CSS, only the bare-bone styles to make it work

Dialog.js:

• is now very thin as the whole open/close logic has moved to Popup.js
• Implements the 2 hooks in the .hbs

Popover.js:

• only wraps open in openBy and has no close since the code is now in Popup.js
• is refactored a bit with several hooks to make it work with the base popup logic
• also implements the 2 hooks in the .hbs

closes: #1837
closes: #1836

[ilhan007]

Really minor, but this "Popup" text would appear in the Dialog API as well. If we consider the term Popup good for the Dialog it's ok, otherwise we should fix it.