Incorrect mouseover graphic for dirty indicator

[per1234]

Describe the bug

The IDE shows a ⬤ on the right side of a tab when it is in an unsaved (AKA "dirty") state.

🐛 When the mouse pointer is moved over this indicator, it turns into an X with polka-dot background. The X is misleading because sketch code tabs are not closeable.

To Reproduce

1. Select File > Preferences from the Arduino IDE menus.
2. Uncheck the box next to "☐ Auto save".
3. Click the OK button.
4. Make a change to the sketch.
   🙂 You should now see a ⬤ on the right side of the tab that has unsaved content.
5. Move the mouse pointer over the ⬤ on the tab.

🐛 There is an X with polka-dot background where the ⬤ is when not moused over:

Expected behavior

No change to the ⬤ dirty indicator when mousing over non-closeable tabs.

Desktop

• OS: Windows 10
• Version: 2.0.0-rc1-snapshot.f0d9894
  Date: 2021-11-30T16:25:41.414Z
  CLI Version: 0.20.1 [abb21449]

[per1234]

Closing as duplicate of #1034, which is perhaps slightly superior due to having a video.