Add indicator for a modified webview that is unsaved

[aematei]

Checklist before submitting a feature request

• I have reviewed CONTRIBUTING.md document
• I have done a quick search on the issue tracker to check if the feature request is not redundant

Is your feature request related to a problem? If yes, describe the problem

Currently, there is no indication to the user when the web view has been modified from the last save state.

Describe the desired feature

Add this indicator based off of what VSCode does to indicate a modified, unsaved file.
Note the "m" on the .fuseki tab of the webview:

Additional context

[aematei]

Might be useful: https://code.visualstudio.com/api/extension-capabilities/theming