Backport Fix #1154: Add @JSExportDescendentClasses (exports subclass ctors)

Backport:
- Fix stub project specifically for 0.5.x.
- Add method `decodedFullName` originally introduced in
  698d69814eb250656f076eda494397e649bb0f78.
- Adapt package name in ExportsTest for 0.5.x:
  scala.scalajs.{testsuite => test}.jsinterop
- Adapt ad-hoc test detection to not "load" classes as in
  698d69814eb250656f076eda494397e649bb0f78
- Fixes #1249 (backport ticket)

(cherry picked from commit b128e480ec8adc6bf1b095de37da725aeebe1c62)

Conflicts:

	stubs/src/main/scala/scala/scalajs/js/annotation/ExportAnnotations.scala