[explorer]: Badge count for dirty editors

[Anasshahidd21]

Feature Description:

We can have a badge count for dirty editors opened, just like vscode. Currently we don't show any badge count for dirty editors.

How to Reproduce:

1. Open a file in Theia
2. Make changes(with autosave off) and without saving, switch to a new file.
3. No count for dirty editors are present.