Revert "Allow changing theme on the fly (#2874)"

This reverts commit 1b73b00b2875c5eafa3c641b77f5d9efc2f7d929 as
well as the related changes to @since, release notes etc.

Conflicts:

	client/src/com/vaadin/client/ResourceLoader.java
	client/src/com/vaadin/client/communication/TranslatedURLReference.java
	client/src/com/vaadin/client/ui/ui/UIConnector.java
	server/src/com/vaadin/ui/UI.java
	shared/src/com/vaadin/shared/ui/ui/UIState.java
	uitest/src/com/vaadin/tests/tb3/AbstractTB3Test.java

Change-Id: Id882dc730f51055f6d17200964bcaf8a1f87a35f