[plug-in] webview compliance

[benoitf]

Fix #4267 by avoiding to define postMessage function through a timeout (now function is defined earlier)

Also:

• keep scrolling of webview when returning back to the webview
• handling keep state when moving webviews
• fix initial webview column
• handle theia-resource and vscode-resource in postMessage content
• handle localRoots
• handle retainContextWhenHidden
• do not let enter messages until the webview is not ready to receive them
• ...

root INFO [hosted-plugin: 60603]   Webview tests
root INFO [hosted-plugin: 60603]     ✓ webviews should be able to send and receive messages (318ms)
root INFO [hosted-plugin: 60603]     ✓ webviews should not have scripts enabled by default (1001ms)
root INFO [hosted-plugin: 60603]     ✓ webviews should update html (93ms)
root INFO [hosted-plugin: 60603]     ✓ webviews should preserve vscode API state when they are root root INFO [hosted-plugin: 60603]     ✓ webviews with retainContextWhenHidden should preserve their context when they are hidden (313ms)
root INFO [hosted-plugin: 60603]     ✓ webviews with retainContextWhenHidden should preserve their page position when hidden (783ms)
root INFO [hosted-plugin: 60603]     ✓ webviews should only be able to load resources from workspace by default (180ms)
root INFO [hosted-plugin: 60603]     ✓ webviews should allow overriding allowed resource paths using localResourceRoots (218ms)
root INFO [hosted-plugin: 60603]     ✓ webviews should have real view column after they are created, #56097 (185ms)
root INFO [hosted-plugin: 60603]    X webviews should preserve their context when they are moved between view columns
root INFO [hosted-plugin: 60603]   9 passing (9s)
root INFO [hosted-plugin: 60603]   1 failing


root INFO [hosted-plugin: 60603]   1) Webview tests webviews should preserve their context when they are moved between view columns:
     Error: Failed to show text document [object Object]
      at Object.<anonymous> (packages/plugin-ext/src/plugin/plugin-context.ts:261:27)
      at step (packages/plugin-ext/lib/plugin/plugin-context.js:47:23)
      at Object.next (packages/plugin-ext/lib/plugin/plugin-context.js:28:53)
      at fulfilled (packages/plugin-ext/lib/plugin/plugin-context.js:19:58)

There is an error on the textDocument (not related to webview itself)

Change-Id: Ie35c911c0c62ebf8260c2449299e7fe91433255b
Signed-off-by: Florent Benoit fbenoit@redhat.com

[akosyakov]

I can verify improvements with VS Code api tests: