Better magic comments at the end of JavaScript fragments in CrossSiteIframeLinker

- Switch to '#' instead of '@' since this changed in the sourcemap spec
- Set sourcemap URL for every fragment, not just the primary one
- Add template variable support to the includeSourceMapUrl config property

Template variables:
  __MODULE__ substitutes the module name
  __HASH__ substitutes the strong name of the fragment
  __FRAGMENT__ substitutes the fragment number

This makes it easier to build a URL that a servlet can resolve to the
right sourcemap file.

Public API changes:
 - New versions of two methods in SelectionScriptLinker
 - Added CrossSiteIframeLinker.getSourceMapUrl()

Change-Id: I2207d2b258df8c1088129017e5039fbfd0edb093
Review-Link: https://gwt-review.googlesource.com/#/c/4891/
3 files changed