RFR: JDK-8266651: Convert Table method parameters from String to Content

[Hannes Wallnöfer]

This changes the parameter type for the tab label from `String` to `Content` in the `formats.html.Table` `addTab` and `setDefaultTab` methods. The change is straightforward because the tab name was converted to `Content` anyway, and all resources are readily available in `Content` form. 

I briefly considered overriding the `hashCode` and `equals` methods in `Text` to be depend on content instead of object identity, but decided against it. This would just cover up bugs where we generate the same content twice. (In the process I made sure we currently don't have any such cases where a `Text` instance is compared to another one with the same string content.)

-------------

Commit messages:
 - JDK-8266651: Convert Table method parameters from String to Content

Changes: https://git.openjdk.java.net/jdk/pull/3994/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3994&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266651
  Stats: 60 lines in 8 files changed: 1 ins; 3 del; 56 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3994.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3994/head:pull/3994

PR: https://git.openjdk.java.net/jdk/pull/3994