Tab menu: show "restart" + maintain items' enable state (#19972)

## Summary of the Pull Request
Changes how/when we display the "restart connection" item in the tab's
context menu. Now, we always display it, but it's disabled if we're not
in a terminal tab.

Applies similar enable/disable logic to the rest of the menu items.
Previous to this, they just wouldn't do anything (which is fair, they
didn't make any sense).

## Validation Steps Performed
Check tab's menu in following scenarios:
✅ terminal pane
✅ settings tab
✅ snippets pane

Closes #18891