BUG: close tabs in tab widget when main window is closed

[shilorigins]

Description

Fix bug where LivePVTableModel polling threads aren't cleaned up when the main window is closed.

Closes #88

Motivation and Context

See #88

How Has This Been Tested?

Interactively

Where Has This Been Documented?

Pre-merge checklist

• Code works interactively
• Code follows the style guide
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions
• Ran docs/pre-release-notes.sh and created a pre-release documentation page
• Pre-release docs include context, functional descriptions, and contributors as appropriate

[tangkong]

While (har) I like the guarantee that a for loop will terminate, this should be sufficient to get all the tabs. If the tab removal process ever somehow doesn't decrement the count, we'll only have ourselves to blame