[manifest] Update for xwalk_launch_screen
[manifest] Crosswalk 8 on Android supports start_url

app.launch.local_path is deprecated on Crosswalk Android 8+
[manifest] csp replaces content_security_policy
[manifest] Make room for orientation page
[manifest] Make field-description page titles consistent
[manifest] Ensure fields are correct for Crosswalk 8
[manifest] Add xwalk_permissions field
[manifest] Flesh out orientation description and page
[manifest] Add W3C-compatible 'icons' field
Remove detail about screen measurement from icons page
Rearrange JSON file order for consistency

*.extension comes after the spec-compatible field in the JSON file.
Update fields in line with comments on XWALK-2131 so far
Update icons page as per w3c spec
[manifest] Add final details of manifest support in Crosswalk 8

Based on information gleaned from:
https://crosswalk-project.org/jira/browse/XWALK-1918
https://crosswalk-project.org/jira/browse/XWALK-890
[manifest] Remove empty sections from xwalk_hosts and csp pages
Merge pull request #138 from crosswalk-project/manifest-xwalk8-2131

Update manifest pages on website for Crosswalk 8
[FAQ] Reorganise into thematic sections