Enable Super Dev Mode hook by default

Set devModeRedirectEnabled to true by default in CrossSiteIframeLinker, and
as a security measure also default devModeUrlWhitelistRegexp to only allow
localhost and 127.0.0.1 on HTTP.

Bug: issue 8413
Change-Id: I9b27419ab69d4f82258429a5d5f258047d33e5ee
Adds ToStringRenderer that uses Object#toString

This adds ToStringRenderer as a Renderer implementation that uses
toString method of the object. This is useful for Strings, boxing
types and any other object that has a toString implementation
suitable for display.

With ToStringRenderer in place, this patch also adds default ctrs
to various Renderer based widgets. (Acutally relying on toString
is the default behavior in all Swing widgets).

Change-Id: If9ea40db1e83f549436fc540f5d0c6a264703a04
Review-Link: https://gwt-review.googlesource.com/#/c/5841/
Merge remote-tracking branch 'upstream/master'