RFR: JDK-8222669: Create and use new html.Entity class

[Jonathan Gibbons]

Please review a relatively simple changeset to introduce and use a new 
class to explicitly handle HTML entities within an HtmlTree. This allows 
for some customized behavior (specifically a character count of 0 for a 
zero-width space) and avoids an otherwise unnecessary use of the RawHtml 
class.

In addition to introducing the new class, the use of Contents.SPACE and 
Contents.ZERO_WIDTH_SPACE is updated to use the new entity constants. 
FInally, explicit uses of strings "<", ">" and "&" are updated 
to use appropriate new entities.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8222669
Webrev: https://bugs.openjdk.java.net/browse/JDK-8222669