Provide addTabs(...) method in TabSheet to add multiple tabs in one method call

-Vaadin 8

Similarly to VerticalLayout's addComponents(...) method, it would be handy to have an addTabs(...) method in TabSheet in order to add multiple tabs in just one call.

Old code would look like this:

TabSheet tabSheet = new TabSheet();
        VerticalLayout tab1 = new VerticalLayout();
        tab1.setCaption("Tab 1");
        VerticalLayout tab2 = new VerticalLayout();
        tab2.setCaption("Tab 2");
        VerticalLayout tab3 = new VerticalLayout();
        tab3.setCaption("Tab 3");
        tabSheet.addTab(tab1);
        tabSheet.addTab(tab2);
        tabSheet.addTab(tab3);

New code would look like this (slightly reducing code lines):

TabSheet tabSheet = new TabSheet();
        VerticalLayout tab1 = new VerticalLayout();
        tab1.setCaption("Tab 1");
        VerticalLayout tab2 = new VerticalLayout();
        tab2.setCaption("Tab 2");
        VerticalLayout tab3 = new VerticalLayout();
        tab3.setCaption("Tab 3");
        tabSheet.addTabs(tab1, tab2, tab3);

Start with TabSheet's existing addTab method and related API behavior. Add an addTabs(...) method that accepts multiple tabs and preserves the behavior of individual additions, then verify that the example call works as intended.