[run on Tizen] Relative paths can be used when installing xpks

Updating as Crosswalk 8 is about to become the new stable
version.

Fixes https://crosswalk-project.org/jira/browse/XWALK-2211
[versions] Crosswalk beta is now 9.38.208.1
[downloads] Update release notes links (Crosswalk 8+9)
[embedding API] Add version 3 API docs