[vscode] support editor/title context menus for web views

[502647092]

Description

preview toolbar also not support
VSCode

Theia

Reproduction Steps

Download vscode plugin REST Client(humao.rest-client)
create a new http file
Send Request
look response

OS and Theia version:
Diagnostics:

[akosyakov]

These actions are contributed via editor/title contribution point for the webview: https://github.com/Huachao/vscode-restclient/blob/ea54abe5e5e54eccd002dbef7ff1693d48983f9d/package.json#L205-L236

The difference between Theia and VS Code is that everything visible in the main area is an editor for VS Code from the perspective of commands and contribution points. In Theia an editor is an textual editor. Right now it's only possible to contribute menu items to textual editor because of it. We should rather base the check on whether a widget is a navigatable or not, i.e. it backed up by a file resource.