[screens] Use window.screen.show() method

For consistency, use the window.screen.show() method in the
explanation and sample code.
[extensions] Add note on building for beta/canary

Fixes https://crosswalk-project.org/jira/browse/XWALK-2170
Merge pull request #144 from crosswalk-project/extension-xwalk-version-2170

[extensions] Add note on building for beta/canary
[extensions] Correctly position note about finding canary version number
Bump stable to version 7.36.154.13
[FAQ] Merge wiki FAQ into main FAQ

Merge content of https://github.com/crosswalk-project/crosswalk-website/wiki/FAQ
into main FAQ on website.

Fixes https://crosswalk-project.org/jira/browse/XWALK-1686
[FAQ] Set headings at correct level
[FAQ] Group sections more intuitively
[FAQ] Remove slightly esoteric section about CEF/QtWebKit