refactor(ui5-popup): remove isOpen, close, show and showAt methods

[TeodorTaushanov]

Removes isOpen and close methods of 'Popup', show method of <ui5-dialog> and showAt method of <ui5-popover>

BREAKING CHANGES:

• isOpen and close methods of Popup are no longer present. Now open property can be used instead.

Previously:

let isOpen = popup.isOpen();
popup.close();

Now:

let isOpen = popup.open;
popup.open = false;

• The show method of <ui5-dialog> and showAt method of <ui5-popover> are no longer present.

Previously:

dialog.show();
popover.showAt(opener);

Now:

dialog.open = true;

or

popover.opener = opener;
popover.open = true;

• parameter preventInitialFocus from methods showAt and show is added as a property.

Previously:

dialog.show(true);
popover.showAt(opener, true);

Now application developers can use:

dialog.preventInitalFocus = true;
dialog.open = true;

or

popover.preventInitalFocus = true;
popover.opener = opener;
popover.open = true;

Related to #8461

[ilhan007]

I think the methods are not removed (Popover.ts)