[plugin-ext]: Modal dialog enhancements

[kittaakos]

Bug Description:

I found a few issues with the modal notification dialog when I was reviewing #8080

• close button should be Cancel. (VS Code uses Cancel),
• The button has to have the accent color, it does not have (try with the light theme, the button is whitish and not bluish),
• The command palette should be disabled when there is a modal dialog,
• Do not hard-code Theia as the dialog title:
  

  theia/packages/plugin-ext/src/main/browser/dialogs/modal-notification.ts

  Line 39 in f5c0827

  super({ title: 'Theia' });

• The electron version should use native dialogs.

Steps to Reproduce:

Additional Information

• Operating System:
• Theia Version:

[tetchel]

to the point about the modal title, obviously it would be nice if extensions could configure the title, but the VS Code API doesn't allow that (there is no title). So since you want to keep your API consistent with theirs, maybe the title could be Information | Warning | Error

[tetchel]

And for the button colour, it's always marked as a 'secondary' button:
https://github.com/eclipse-theia/theia/blob/master/packages/core/src/browser/dialogs.ts#L198

but it should only be 'secondary' if at least one other button is present on the modal.

[vince-fugnitto]

to the point about the modal title, obviously it would be nice if extensions could configure the title, but the VS Code API doesn't allow that (there is no title). So since you want to keep your API consistent with theirs, maybe the title could be Information | Warning | Error

@tetchel for the title, I believe the idea would be to use the applicationName (similarly to other places) and not hardcode the value 'Theia'.

protected applicationName = FrontendApplicationConfigProvider.get().applicationName;