[HCR] Action button to apply code changes to running app

[akaroml]

The UX of HCR can be improved by making it explicit. With the latest update of VS Code, you can now add a button in the debug toolbar to expose this feature. And this is how Dart guys did:

Please refer to this issue for more details: microsoft/vscode#69153

[testforstephen]

Add a new debug settings java.debug.settings.hotCodeReplace to control how to trigger HCR. Defaults to manual.

• manual - Click the toolbar to apply the changes.
  
• auto - Automatically apply the changes after compilation. This is the old behavior.
• never - Never apply the changes.