Revised the setNewItemsAllowed() documentation. setImmediate(), NewItemHandler, etc. Some general selection component revision. #13485

Change-Id: Iab54de62a23f68330a35001ab52eff88e7eb78f5
Minor fixes to theme chapter. #12697

Change-Id: I40b7651ced511029e7f1adc593f156919811fba1
Mention Liferay theme, and minor fixes. #12697

Change-Id: I6d7f03e2b18cf8c43cdcffb2bc9af1359c1d73b9
Merge branch 'master' of ssh://dev.vaadin.com:29418/book-of-vaadin into testbench-4.0