ui: Migrate CheckBoxes to use binary state functions setChecked/isChecked

[joshanne]

Various points in the code make use of Tri-state functionality (ie. setCheckState()/checkState()).

In working to migrate the tool to PyQt6, the checkState() returns and enum, and the enum CheckedState.Unchecked is not considered Falsy.

What that means is if chkbox.checkState() always returns True.

The checkState functionality allows us to make use of a PartiallyChecked state, which is never used inside the codebase - so let's just use the binary states.

# In PyQt6...
>>> from PyQt6.QtCore import Qt
>>> print(f"{bool(Qt.CheckState.Unchecked)=}")
bool(Qt.CheckState.Unchecked)=True
>>> print(f"{bool(Qt.CheckState.PartiallyChecked)=}")
bool(Qt.CheckState.PartiallyChecked)=True
>>> print(f"{bool(Qt.CheckState.Checked)=}")
bool(Qt.CheckState.Checked)=True

This differs from PyQt5 where:

# In PyQt5...
>>> import PyQt5
>>> from PyQt5.QtCore import Qt
>>> print(f"{bool(Qt.CheckState.Unchecked)=}")
bool(Qt.CheckState.Unchecked)=False
>>> print(f"{bool(Qt.CheckState.PartiallyChecked)=}")
bool(Qt.CheckState.PartiallyChecked)=True
>>> print(f"{bool(Qt.CheckState.Checked)=}")
bool(Qt.CheckState.Checked)=True

Needed to resolve Issue #107